Koi is a really sweet fish, but can get aggressive easily. She does not talk and only makes grunting noises.
Koi is on the school basketball team, and seems to be the best out of all the team players, since she is the biggest and strongest out of all of them ("Mascotastrophe").
In the "Potatoes for Winter" play, Koi played as the narrator. Her mom cried when she said her lines. Her main role in the play was "Yak #3", who was used for Bea's transportation ("Fish School Musical"). When the gang all decided to go trick-or-treating outside the tanks, Koi's costume was a horse. Finberley was the front and Koi was the back. When Jocktopus took off with everyone's candy, everyone decided to give up, making Koi wrap Steve Jackson around her arms and start crying ("Halloween Haul").
Koi had her biggest role in "Koi Story", when she went to a slumber party at Bea's house and it was revealed that she was the only one in the group who had never done anything crazy. She lied, however, and told thegirls she did do something crazy, though it is not understood what she actually said because she cannot talk. After Bea and the girls went to Bea's kitchen to get macarons, Koi saw Oscar and Milo in the window and told them she was upset that she lied. Milo told her it is okay to lie. When Koi went into school the next day, everyone was staring at her in admiration because the girls told everyone what she did. Koi eventually gained confidence in herself, but when Piranhica accused Koi of being a liar, Koi said she was not and agreed to jump a cat, immediately regretting it. She then found Milo and Oscar and told them what happened. Milo was shocked that Koi had not told the truth because he thought that by "lying", she meant lying down. Milo told her to stand by her word and do it, but then regretted telling her after he saw the cat and the jump she was about to make. Koi confessed to lying, but Piranhica pushed her onto the ramp, and she made the jump. Koi survived, getting half of her face bitten off, and told the girls she was sorry.