Koi is a koi fish. She's the second biggest fish in Freshwater High, the first one is Jocktopus and the third is Jumbo Shrimp.
Even though she's really sweet, Koi could get mad easily. She makes jokes that makes other fish laugh and say "oh, Koi!" She doesn't talk and she makes grunting noises.
She was invited to Bea's sleepover party and brought triplets to the party. When Clamantha suggested painting nails, Koi got out a box of nails. The other girls laughed. Koi was mad when Milo and Oscar disguised as girls. She picked them up and dunked them on mud, overfed them with dip, and tortured them, along with the girls ("Fish Sleepover Party"). Bea was talking to Koi, Milo, Oscar, Shellsea and Finberley in her laptop. It was interrupted by Bea's mom. She was waiting for the school bus and saw Bea as an adult ("Bea Becomes an Adult Fish"). When the board said the school dance is "tomorrow", Koi made it say "today". She helped Bea become queen of the dance ("Queen Bea"). When Milo got a ninja, Finberley was about to tell Koi what Steve Jackson said. She leaned over to whisper to Koi, but Milo was in between them and put them in a bowl for telling secrets ("Milo Gets a Ninja").
Koi and Shellsea gave Bea some advice on how to make Albert Glass like Bea ("Fishing for Compliments: The Albert Glass Story"). Koi and the rest of the girls laughed at Milo when Coach Salmons said that Milo is much less stronger than the girls. When a giant Milo returned to Freshwater after staying in the Big Tank too long, Koi and all of the students ran away, not knowing it is Milo ("Big Fish"). Koi was part of the girl's team at the Homecoming float competition. Koi helped the girls make their float better than the boy's float ("Fish Floaters"). Esmargot and Koi received Milo's invitation to a "Cinco De Milo" party, which is actually a scam by Milo to lure everyone away from the tanks ("Pamela Hamster Returns"). Koi is on the school basketball team, and seems to be the best out of the team, since she's the biggest and strongest. At the end, Freshwater High ended up beating the game against the geckos, thanks to the school's mascot ("Mascotastrophe").
When Bea spiced things up for Oscar's morning announcements, she put on a segment which has Koi throwing Clamantha down a grid, and whoever has the same number that she lands on is the winner. When the cafeteria shut down, Koi went crazy for food like the rest of the students. She and Jocktopus also pushed the statue of the diver that's in front of the school ("Good Morning, Freshwater"). When the Freshwater High cheerleaders squad disappeared after getting swooped out of the tank by Bud, Bea called the girls and asked them to join the cheer squad to compete against Clamantha's rival, Clamanda. The new cheer squad practiced on the school's football field. After many attempts to preform a perfect routine, Koi and the cheer squad eventually mamde it after Oscar changed the whole routine. At the day of the cheer-off, Freshwater's cheer squad won for having the best spirit ("We've Got Fish Spirit"). Like the rest of the students, Koi was really excited when the class was having a field trip to Pupu Goodtimes Amusement Park. Koi and the girls gave Bea advice when she was about to ride a rollercoaster with Steve Jackson. Koi and the rest of the girls (except for Shellsea and Bea), spent their time buying food and souvenirs ("Good Times at Pupu Goodtimes"). Koi was in the classroom when Mr. Baldwin took attendance. Koi also cheered for Oscar along with everyone else when he finally won the perfect attendance award ("Run, Oscar, Run").
In the "Potatoes for Winter" play, Koi played as the narrator. Her mom cried when Koi reported the narration. Koi's main role in the play was Yak #3, who was used for Bea's transportation. The yak died, however, when Bea started the journey to save the Great Potato (Milo) ("Fish School Musical"). When the gang all decided to go trick-or-treating outside the tank, 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 and Finberley worked together to try to win the talent show. At the end, the two didn't even perform yet ("Fish Talent Show"). According to Oscar, Koi and the girls watch Steve Jackson unpack every morning. When Oscar, Milo, Albert and Jumbo fought for Steve Jackson's hair gel, they spilled the gel all over many student's hairs, including Koi ("Hairanoid"). Although not shown, Koi didn't make it to the marching band when she played the accordion to Lonnie at band tryouts. At the battle between Milo and Lonnie, Koi and all of the students who got rejected fought Lonnie's band ("Banned Band"). Koi was ice skating when she received Milo's party inviation. At Milo's Christmas party, Koi was skating around the Christmas tree with the other students of Freshwater ("Merry Fishmas, Milo").
- Her name is briefly seen in the yearbook at the end of "Bea Stays in the Picture".
- Koi is voiced by Rachel Dratch, who provides the grunting noises.
- She seems to shrink enormously than in her first appearance in "Bea Stays in the Picture"
- She has a Fishbook account and has a chat tank ("Bea Becomes an Adult Fish")
- She is one of the basketball players ("Mascotastrophe")
- Koi doesn't have a full set of hair like Bea, her only hair is a ponytail.
- She is one of two fish to not have the bumpy tail design (Steve Jackson)
- Koi has been able to say a few mumbled words, such as "Better", etc. This is similar to Happy Tree Friends, where their words are gibberish most of the time but sometimes speak clear English, still garbled a little.