Oishi Kawaii (pronounced oh-we-she ka-wa-ee), born January 1, 1996, is the main protagonist of Oishi High School Battle. She takes up demon-slaying after her father, who was fired from the job. Oishi tries to defend the world against many demons that inhabit Earth all the while trying to survive high school.
- Ian Hecox and Anthony Padilla: After the two made fun of Oishi's outfit, she appeared and stabbed Ian in the chest with her knives. Before attacking Anthony, he apologized and Oishi forgave him and gave him a kiss. Noodles later ate Anthony.
- Ryuzu Kawaii: Oishi's father. After failing at his job as a demon slayer, he was fired and Oishi replaced his position. Oishi thinks of him as the best demon hunter in history and still does, even if the kids at her old school say otherwise. Worried that Ryuzu's ruined reputation will cause her to get an ugly spirit animal, Oishi and Mucusance form a plan to make Ryuzu a hero again, only to fail miserably. At the ceremony where the students get their spirit animals, Ryuzu accidentally damages the portal, which ends up having Oishi get Noodles, the exact opposite of what she was worried about. Ryuzu comes to Oishi's Career Day, but is humiliated by Raif, and then later dates Oishi's teacher. Sometime after this, Ryuzu learns of Oishi's sexting to Bobby Johnson, and teleports through Bobby's phone to finish him. However, Bobby's phone runs out of minutes and Ryuzu is teleported back vowing to get revenge. Ryuzu kills Bobby 2 years later. Ryuzu tries to make Oishi move to another dimension, but is defeated by her in a love battle.
- Mucusance: Oishi's boyfriend (presumably ex-boyfriend as of Teen Pregnancy) and enemy of Raif. Oishi is marrying him because his race will end if he does not marry her. Mucasance eats Raif after beating him in a Dance Battle for Oishi's affection. During Oishi Origins, Mucusance tries to get her to participate in tentacle rape-related activities, but Oishi, still worried about Ryuzu's reputation, keeps getting him off track of his original plans. As of Teen Pregnancy, they are assumed to have broken up, since Oishi complains that Mucusance took her for granted and says that she learned what makes a good boyfriend in Earth.
- Raif: Captain of the football team at Oishi's High School. He tries to date her, but is eaten after losing a dance battle. He is seen on THE GAY KID!. He makes fun of Oishi's father in CAREER DAY!. During the course of season two, he continually tries to win Oishi's heart but fails. However, he manages to successfully do so in The Final Battle, which in turn allowed Oishi to perform the Kawaiimeha Blast, her strongest attack.
- Bobby Johnson: Boy from Oishi's High School who has a crush on her. Oishi saves him from Heaven after being hooked up with him in a spin the bottle match. Later, Dakota convinces Oishi to sext Bobby, but when he receives the messages, he throws up and becomes emotionally unstable. When Oishi's father finds the pictures, he tries to kill Bobby but is sucked back into Bobby's phone after it runs out of minutes. He is also Oishi's date for the homecoming dance in Homecoming Dance, but was eaten by Mucusance, where he is still undigested inside of him as of Oishi Has Boy Trouble.
- Aubrey: One of Oishi's friends at schoool. He is a homosexual. He is also on the football team, but the team does not suspect his orientation. When Oishi is told that Aubrey has his own inner demons to battle, she sneaks into his bedroom one night to slay the demons. Aubrey wakes up and tells her that some demons were meant to be accepted.
- Ms. Hooper: Oishi's English Literature teacher. She dated Oishi's father, Ryuzu, until Oishi trapped her in a deadly crystal ball which repeatedly blew her up. She survived the ordeal and continued to date Ryuzu until he told Oishi they were moving. In Homecoming Dance, it seems that Oishi and Ms. Hooper are in better terms.
- Dakota: One of Oishi's good friends at school, that has repeatedly tried to hook Oishi up with Bobby Johnson. Dakota likes to give advice to Oishi, and sometimes tells her things about high school. Raif likes her. Dakota is in love with Noodles, until Ryuzu attacks her during Oishi's love battle. It is unknown what happened to her after season 1, or if her character has completely changed to Tara by the writers during the transition to season 2.
- Mere-Death: Goth girl at Oishi's school. Oishi feels sympathy for her for cutting herself, and asks Mere-Death if she wants to die. Mere-Death runs away, but Oishi later kills her in her backyard, while Noodles eats cookies that Mere-Death's Mom made.
- Noodles: Noodles is Oishi's dog. Noodles almost never leaves her side, and protects her. Dakota loves Noodles, and Noodles tries to impregnate her in Moving Away.
- Overlord Dave: Oishi's High School Principal. He gives Oishi advice in some episodes, including how to deal with her substitute teacher and the cool girls that run the bathroom. However, the advice has been really unhelpful, in the case of the former, or taken the wrong way, in the case of the latter. Principal Dave likes to talk to a boy in a wheel chair that attends the school.
- Tara: Another one of Oishi's good friends. In the beginning of season 2, she states that she took care of Noodles while Oishi was at her mom's apartment only to be the first of many girls impregnated with Noodles's offspring. Since then, she has been recovering from having her face blown off after the birth of the offspring.
- Twin Kunai - This is Oishi's primary weapon she uses for battle.
- Battle Breasts - Oishi's signature move. It was shown only once in season one showing that it's capable of shooting lasers. In season two however, she is allowed to freely manipulate her breasts to do attacks, such as punching.
- Ultimate Form - Oishi's transformation allows her to grow a third breast.
- Kawaiimeha Blast - Oishi's strongest attack, but she can only use this attack once she has found true love. During this attack, Oishi shoots a laser that summons several cute creatures, enamoring even the most evil of villains. However, the creatures then transform into fearsome beasts. The name of the attack is a spoof of Dragon Balls' Kamehameha.
- OISHI WILL KICK YOUR ASS!
- The Crush (Oishi High School Battle)
- Substitute Teacher (Oishi High School Battle)
- Sleepover (Oishi High School Battle)
- The Gay Kid (Oishi High School Battle)
- Career Day (Oishi High School Battle)
- Sexting (Oishi High School Battle)
- Detention (Oishi High School Battle)
- Cutting (Oishi High School Battle)
- Cool Girls (Oishi High School Battle)
- Moving Away (Oishi High School Battle)
- The Greatest Warrior (Oishi Origins)
- The Helping Hand (Oishi Origins)
- The Ceremony (Oishi Origins)
- IPhone 5 REVEALED (cameo)
- The End of Christmas (Part 1) (cameo)