Ester is the Starfish Princess and the best friend of Polvina and Tubarina, though she sometimes seems to get along better with the former. She usually visits the Octopus and Shark Palaces from time to time, especially after school and on weekends.
Ester's personality changes according to the situation. When she makes plans, she assesses the ideas and plans of others and adapts them or does it her own way with less-than-honest actions.
Ester is a very curious girl who loves to read and watch documentaries on cable television. She dreams to one day know about Dryland and also the stars in the sky. She also has a pet starfish named Spikey.
She has brown hair with a yellow and orange starfish cap, along with a matching top and skirt.
In The Golden Penguins, Ester finds in the sand a pair of golden penguin earrings that belonged to the Penguin Queen, who had dropped them while picking Isa up from school, and takes them home. As she expresses her interest in keeping the earrings, Ester finds herself tormented by the earrings seemingly speaking to her, only to be told that it is actually her conscience talking. Eventually, Ester returns the earrings to the Penguin Queen and is told that they are supposedly magical as they talk to some people.
In The Babysitters, Ester and Marli decide to sign up for the same job babysitting the Whale Queen’s son, Naimo, much to their chagrin as the two have a rivalry with each other. Unbeknownst to the Whale Queen, Naimo has gone searching for his toy whale, Cici, and they search around the Whale Palace for him. Following this, the Whale Queen recommends Ester and Marli to other people after the good job they did looking after Naimo.
While Ester is preparing for the end of term class party in The Party, she borrows her father’s Star Pen when her pen runs out and she realises that she forgot to bring her pencil case home. When the pen breaks, Ester tries everything to fix it while trying to evade her father’s questioning. Eventually, she decides to bury the pen outside the palace. Her father discovers this and confronts Ester the next morning. Upset with having been lied to, the Starfish King decides to punish Ester by not allowing her to attend the class party. When Polvina and Tubarina hear about it, they decide to convince the Starfish King to let Ester attend the party; when they find a pen restorer who can fix the Star Pen, the Starfish King allows Ester to attend the party as long as she stays in her room.
When Marcello introduces his friend, Caton, to the girls in The Crush, Ester falls in love with him at first sight, which is strange as Ester is not usually interested in love and romance, believing that love stories are ridiculous. She becomes obsessed with Caton to the point that she is unable to see him for who he really is and realise that he is treating her like a slave, which is something that Ester is usually good at. Polvina and Tubarina notice this and decide to help Ester see Caton for who he really is. Though their initial attempts at using Gummy to do this fail, Ester eventually sees Caton being mean to Gummy and swears off falling in love with boys ever again (at least until she grows up).
While practising for a shellsled race against the boys in Ester's Fear, Ester crashes the sled while trying out a special move and becomes afraid of riding it again. She struggles between hiding from the girls and not letting the girls win; when Polvina and Tubarina hear about this, they tell Ester how they helped them overcome their respective fears of monsters under the bed and scary things in the cellar. Polvina offers to steer the sled for Ester in the race, but when she is unable to go faster, Ester is reminded of the prospect of Marcello doing his victory dance and decides to take over. She does her special move again and they land safely, winning the race. Polvina reveals to Ester that she went slow on purpose because she and Polvina knew that her fear of the boys winning was worse than her fear of riding the sled.
In The Runaway Grandmother, Ester's grandmother visits for Salacia Day with a red beanie. Believing that it is "totally uncute", she "loses" the beanie and wears a headband. The Starfish Grandmother sees this and is saddened, but when Ester goes after her to apologise, she finds her grandmother's red scarf handing on a cliff to the Abysmal Kingdom, Ester believes that she fell down and the girls head down to look for her. While doing so, the girls are chased by an anglerfish, but when Ester's beanie falls out of her bag, it becomes fascinated by it. After finding Bia and having no luck finding Ester's grandmother, Ester gives the beanie to the anglerfish. When they return home, they find that the Starfish Grandmother had returned to the Starfish Palace, having lost her scarf when the current took it away. Ester tells her grandmother that she likes the beanies again because it saved them; it turns out that her grandmother had made some more beanies, which she gives to the girls.
When Ester gets shown up on the seaball field by Leia, who had never played seaball before, in Princess Poutalot, Ester becomes determined to prove herself the best player, only to get shown up again by Leia. She decides to swear off playing the game, at which point Polvina and Tubarina begin devising a plan to get her to play. They initially play a game at the Starfish Palace, but Ester sees through them and gets Spikey and her friends to throw balls at everyone. Seeing this, Polvina and Tubarina decide to show Ester that nobody would care if she played with them or not. After seeing this, Ester eventually decides to join everyone in playing seaball again, saying that she would rather be second best and have fun than have no fun at all.
In The Itchy Wrists, Ester asks her parents for a skin painting and they refuse. When Polvina and Tubarina refuse to do one for her, Vivi offers to do so with a skin painting maker that she had, which Ester accepts. Though Ester manages to hide her skin painting from her mother, the Starfish Queen notices her scratching her wrist. The next day, they discover that Vivi gave a few other girls skin paintings as they see them scratching their wrists. Polvina and Tubarina decide to scrub off the painting, which leads to a chase that causes Ester to bump into Big Nose Rock. Ester wakes up in her bedroom, where she discovers that her mother scrubbed off her skin painting and she is punished as a result.
In The Ester Breakout, the Starfish King discovers Ester showing her friends to a Drylander lighthouse and is grounded. After the second day, Ester gets bored and decides to head to the lighthouse just as a large carriage crashes on a reef near it. Noticing black goop coming out of the carriage where the reef tore it, Ester tells the fish to get away before she plugs up the hole. When initial attempts fail, Ester jams the seaweed in with her rear end, but she gets stuck with nobody around to help her. Polvina, Tubarina and the Starfish King find her the next day, after they notice that she hasn’t come to school that day, and take her back home just as more Drylanders come to rescue the carriage. The Starfish King punishes Ester for disobeying him, but allows her to play with her friends for the day.
In The Healer, Ester meets a mysterious boy after seeing Bib feeling unwell, but he disappears just as soon as he appears. She tells Polvina and Tubarina about him, but they are unable to believe her because she is unable to tell them who he is. The next couple of times Ester meets the boy, he evades her questions and when Ester tries to show Polvina and Tubarina where he lives, he had already left. With no other choice, Ester decides to use Spikey to trick the boy into appearing. Polvina and Tubarina are finally able to meet him, but the boy is saddened at being tricked into doing so and Spikey is angry with her to the point that he decides to travel with the boy, only to be rejected. Ester reconciles with Spikey and the boy tells her that while they could not be friends now, he never said that it would be forever. As the boy leaves, he reveals his name to Ester as Duante.
In the books
In The World of Salacia, Ester is excited for the first day of school. She comforts a nervous Polvina as they go to class together and recognises that Tubarina will become a problem for them after Polvina accidentally offends her.
At school, Ester becomes curious about exploring Dryland after hearing about it from Miss Marla. After school, she convinces Polvina to swim to the limits of Salacia together to see what Dryland is like. Upon reaching the surface, they see a group of Drylander hunters attempting to capture a shark named Mr Dennis. As Polvina swims back to get her friend, Mr Mollusc, for help, Ester grabs onto Mr Dennis, who to her surprise, can understand the starfish language. Polvina returns with Mr Mollusc and they pull on Mr Dennis together to prevent the Drylanders from capturing him until Tubarina arrives with a group of sharks, who break Mr Dennis free and scare off the Drylanders.
Upon returning home, Polvina and Ester reconcile with Tubarina and become friends with her. When the Octopus and Shark Kings heard about their adventure, Ester was presumably punished by having her allowance suspended for four months. However, their story spread through all the kingdoms, leading to them being featured on the cover of Scales magazine.
In A Shadow in the Water, Ester hears about the opening of Sea Park and becomes excited about it to the point that she talks about it endlessly to Polvina, much to Tubarina's chagrin. She becomes overjoyed when Miss Marla announces that the whole class will be going there, but blows it the day before the trip when she gets into an argument with Tubarina and Maurico, leading to them being grounded from going to Sea Park and having to stay at school instead. Though Polvina offers to stay with her friends, who are being looked after by Mr Lorenzo, Ester is saddened at not being able to go to Sea Park.
When a cloud of black goop covers Salacia and the Shark King has the camouflage activated, the group and Mr Lorenzo find themselves locked within the school as a Drylander diver appears to close in them. Tubarina breaks a window and heads out to rescue Felipe, a baby shark, from whom they learn that the Drylander was actually trying to help him, not hunt him.
As Mr Lorenzo offers to take Felipe up to Dryland, Polvina and the others go to warn the Shark King before he sends the sharks to attack the Drylanders. After gluing sand on some cut-up rugs to conceal themselves from the Drylanders, Ester and Tubarina head to the Shark Kingdom to find the Shark King. Tubarina manages to commandeer some sharks stationed there and head to where Miss Marla and the others are. They allow the animals affected by the black goop to be taken up to the Drylanders, knowing that there are not enough resources in Salacia available to help everyone.
Over the next few weeks, life gradually returns to normal in Salacia. When Felipe returns and tells the girls and Maurico that Mr Lorenzo had "pardoned" them of their punishment, Ester is finally able to go to Sea Park with her friends again.
In The Windy Letters, the girls and Tata head to an antiques fair, where they buy from an old man named Weasel a letter in a bottle that came from Dryland. Upon having the letter translated by Miss Marla, they learn about a poor Drylander girl named Windy who is wondering about what is under the sea.
Ester becomes enthusiastic about going back to Weasel to buy more letters, which is met by opposition from the other girls, but she eventually convinces them to agree. After agreeing to meet Weasel again with his partner, Mr Chain, the girls find themselves kidnapped and taken off-road. Polvina gets the idea to use an oxygen tank to blow themselves out of the carriage. Before they do, Tata gives Ester one of the letters that were in the carriage, but she accidentally lets go of it and it falls into the Abysmal Kingdom.
When the second letter they salvaged leaves them with more questions than answers, Ester goes with Polvina and Tata into the Abysmal Kingdom in an attempt to salvage the third letter. After being forced to separate when a school of abysmal fish close in on them, Ester is saved by Tubarina and Mr Gilson. Upon reuniting with Polvina and meeting Bia, the Abysmal Princess, the girls learn that Mr Chain and Weasel had taken Tata hostage. However, thanks to Bia and Mr Gilson, they manage to find Tata.
After having dinner with Bia in the Abysmal Palace, the girls learn that Bia had managed to salvage the third letter and pass it on to Miss Marla for them. The revelation of Windy's father being a turtle hunter leads to Polvina and Ester comforting a distraught Tata.
Later, Polvina and Ester are with Tata in the Turtle Palace upon learning about Tubarina's disappearance after school that day. Tubarina is brought in by a turtle named Mrs Constanza, who had attacked her after smelling on her the scents of Mr Chain and Weasel, who had been robbed of the three letters she had in her collection weeks earlier. Tubarina had managed to take the fourth letter and Miss Marla translates it to them, but not before having the girls promise not to look for any more letters.
When Polvina manages to connect her ballad to Thorn, the Shadow King, the girls head to the library in an effort to check the royal family records, only for the librarian, Mr Lombada, to bar them from doing so as they did not have authorisation. However, their trip was not all for naught as Polvina discovered a book Miss Marla wrote about the famous objects of Salacia. From that book, the girls learn about the Staff of Oblivion, which was used to make people forgotten by others, and conclude that someone must have used it to make them forget about the Forgotten Princess, the main subject of the ballad.
That night, Ester and Tubarina offer to sleep over with Polvina. Maya, a civilian girl, visits and offends them with her attitude, which leads her to leave. The next morning, the girls learn that the library was robbed just as they were planning to go there after getting the authorisation to browse the royal family records. Realising that the Forgotten Princess couldn't be someone in their class, the girls go to see Miss Marla, only to find that some sharks had broken in and taken her class diaries. However, for some reason, Miss Marla had separated the student reports and put them somewhere else. From the reports, they discover that the Forgotten Princess is actually Maya, or rather, Bia, the Abysmal Princess.
Just as the girls are preparing to take action, Miss Marla prevents them from leaving, not wanting them to get involved. However, Tubarina tricks Miss Marla into letting her outside, at which point she calls for Gummy and goes to find Bia, leaving Polvina, Ester and Miss Marla to alert the Shark King. Upon doing so, Polvina and Ester are warned to stay behind, but an overenthusiastic Ester finds a great white shark, intending to follow the Shark King and Miss Marla into the Abysmal Kingdom.
Amidst the standoff between the Shark King and Thorn, Polvina and Ester rush in and knock Thorn down, taking the Staff of Oblivion away from him, only to be thwarted by a giant octopus. The octopus squeezes Polvina, Ester and the shark, only to let go when he is scolded by Polvina. Thorn uses the Staff to make everyone recognise him as the true King of Salacia, but is thwarted when the Staff destroyed his mind due to him not being the guardian of the staff, which Bia was. Thorn was arrested soon after.
At the end of the year, the girls and Bia went on a holiday trip to the Abysmal Kingdom, visiting children like Maya and having more risky adventures.
In Turtles in Danger, Ester and her starfish friends work with Polvina and her octopus friends to gather up the plastic bags floating around in the sea.
In The Little Hero, Ester is tracing Spikey on a sheet of paper when Polvina and Tubarina appear with Tentie and Gummy, inviting her to a race together. Ester tells the girls that she can't ride Spikey because he is too small, leading to an argument with Tubarina, who calls Spikey "useless". When Leia asks them for help freeing a whale from a Drylander net, Ester uses Spikey to free Tentie and Gummy when they become trapped while trying to free the whale themselves. The whale is freed and everyone proceeds to ride the whale together.
- In the first three main series books, Ester has been known to affectionately call Polvina "prima"/"cousin" (changed to "sis" in the translation) on account of their close relationship with each other.
- According to The Ballad of the Forgotten Princess, Ester's birthday is in September. Given that her zodiac sign is Virgo, it can be presumed that her birthday falls within the range of 1-22 September.