Tapu Fini has a black torso and light blue hair. It has a white headband-like marking on its head and a white V mark on its chest. To the sides of this V mark are two violet shell-like extensions from which protrude two arms, each with three webbed fingers that end in violet claws. It has a fin at the lower front and does not appear to have legs, but an oval shaped platform of sorts on which it stands. Its eyes have light blue eyeshadow-like marks and are light cyan with white eyebrows of sorts. Its expression is often listless, even as it unleashes moves in battle; some attribute this to its moves being exhausting to use. Its shell is purple with a fin on top and a horn—when closed, this shell resembles a swordfish.
As the guardian of Poni Island, Tapu Fini can freely manipulate water. To avoid harm to itself in battle, it shrouds its surroundings in a mysterious fog that sends its opponents into a self-destructive trance. The guardian also unleashes this fog as a test for those who seek the special water it can create, which is said to purify anything it touches. Many humans have attempted this trial, including those with ill intent; disgusted by this history, Tapu Fini does not care much for humans. Despite this, it is willing to help those who prove themselves worthy. As shown in the anime, Tapu Fini has shown to be able to make water clocks out of water and bridge the world of the living with the world of the dead using mist. Long periods of exposure to this mist can trap people within it forever, specifically targeting those who cannot endure the pain of seeing a deceased loved one again.
Tapu Fini's appearance when it closes its shell resembles a swordfish or a marlin. When its shell is open, Tapu Fini resembles a mermaid or Siren. It may also be inspired by Kanaloa, one of the four major Hawaiian gods and god of the ocean. Kanaloa's association with the underworld may inspire the Ruins of Hope, Tapu Fini's shrine, said to be where "life ends its journey".
Tapu Fini may be a combination of tapu (Hawaiian for sacred or holy) and fin. It may also derive from finish or final, referencing the fact that the Ruins of Hope are where "life ends its journey", and also, possibly, that Poni Island is the final island in the island challenge.
Kapu-Rehire may be a combination of kapu (Hawaiian for sacred or holy) and 鰭 hire (fin).