Delfi is the Dolphin Princess, who once travelled for many months with her parents. After her big journey, she received a big party when she returned to school. During her travels, Delfi saw amazing things in Salacia and on Dryland, which her parents requested to keep secret. Because of this, no one knows what Delfi saw, not even the curious Ester. Maybe it's better this way...
In A Weighty Problem, the girls enlist Delfi's help to help Gummy get some exercise. Tubarina and Delfi have Gummy accompany Ping and Patchy so he can do so, but when the two dolphins get caught in a drift net, Gummy returns home to get help from the princesses. The girls free Ping and Patchy, allowing them to summon their pod and help Gummy, who was also caught in a net while trying to distract the Drylanders who were behind the drift net.
In The Great Escape, Delfi and the girls are playing with Ping and Patchy when they meet another dolphin named Nub. When Nub leads Ping and Patchy to the Drylanders who attempted to capture them before, the girls follow them to an aquarium, which Delfi describes as a "big cage for dolphins" where Drylanders teach tricks to them for the entertainment of other Drylanders; she can never understand why they would want to do so. The next morning, the girls sneak into the aquarium, where they find Ping and Patchy captured along with Nub and two other dolphins. Delfi stays with the dolphins while the girls work out how to unlock the gates so they can escape. When Nub decides to escape with the other dolphins as well, Delfi sees off Nub as he joins Ping and Patchy's pod.
In The Sick Dolphin, Delfi is shocked when the girls "rescue" Misty, a dolphin who has trouble digesting her food, as she was being studied by two Drylander scientists who were researching dolphins and looking after Misty in an attempt to make her better; Delfi is unable to help her because she doesn't know what the scientists are giving her. When the girls attempt to bring Delfi and Misty to the scientists, they see their boat sailing away into the distance, forcing them to wait until they return. After trying to search for the scientists to no avail, the girls decide to wait with them as well. When Ester becomes impatient, the girls set off in search of some brown algae, as suggested by Delfi herself when asked what medicine she would give Misty. After the girls return with the brown algae, the scientists return and Delfi has Misty go back to them while she finds out what medicine the scientists are giving to Misty. Once Delfi is satisfied with what she finds, she parts with the girls to keep an eye on Misty's recovery.
Delfi is also mentioned in Deep Freeze when the girls ask her for a favour to borrow some dolphins so they and Isa can play with them while her parents are at the penguin colony.
- In the animated series, Delfi's dolphin crown and hair changes direction depending on which way she is looking.
- Delfi is an exclusive character to the animated series, thus she does not appear in the books.