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, Delfi has Gummy accompany Ping and Patchy so he can get some exercise. While she goes to have lunch with Polvina, Ester and Tubarina, Ping and Patchy find themselves caught in a drift net, leading Gummy to return home in order 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 behind the drift net.
Later, in The Great Escape, Delfi, Polvina, Ester and Tubarina are playing with Ping and Patchy when they meet another dolphin, Nub. When Nub leads Ping and Patchy to the Drylanders who attempted to capture them in the previous story, the girls follow them to an aquarium, where Delfi describes it 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 Polvina, Ester and Tubarina 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 Polvina, Ester and Tubarina "rescue" Misty, a dolphin who has trouble digesting her food, as she was being studied by two Drylander scientists who were researching dolphins and look 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 three 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, Polvina, Ester and Tubarina decide to wait with them as well. When the three become impatient, they decide to find some brown algae, as suggested by Delfi herself when asked what medicine she would give a dolphin. After the three 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 Polvina, Ester and Tubarina to keep an eye on Misty's recovery.
Delfi is 8 years old and has the Zodiac of Pisces (♓).