Megumi Eto is a minor character in the story, designated as Girl number three among the forty-two students forced into the deadly program. She attends Shiroiwa Junior High School as a third-year student in Class B. Megumi comes from a normal family that includes her parents and an older sister; rumors circulating at school falsely label her sister as promiscuous, but Megumi knows those rumors are untrue. In the original novel and manga, she is friends with Mizuho Inada and Kaori Minami, while in the film adaptation her closest friend is Noriko Nakagawa. She harbors a romantic interest in Shuya Nanahara in the novel and manga, whereas in the film her affections are directed toward Shinji Mimura. Megumi enjoys photography in the film version, taking pictures with a Polaroid camera, including snapshots of Mimura and herself, as well as a shot of Shuya, Noriko, and Yoshitoki Kuninobu on the bus before the game begins. In the novel and manga, she carries a cellular phone given to her by her parents so she could contact them if anything went wrong during the school trip.

Megumi is depicted as a cute, playful, and caring girl who deeply values her friendships and her crush. Her primary flaw is that she is overly trusting of others, which proves fatal. She is also the victim of persistent bullying from Hirono Shimizu, who would attack her or cut her skirt with a razor in the hallways. Although Hirono loses interest in tormenting her shortly before the program begins, Megumi remains wary of her and the group of girls Mitsuko Souma leads. When the game starts, Megumi is terrified and immediately seeks refuge in an abandoned house in the residential zone. She decides to use her cell phone to call her parents for rescue, believing that the military cannot track mobile lines. However, the call is intercepted by the teacher, who mocks her and demonstrates that all communications are monitored. While hiding under a kitchen table, she hears someone enter the house and recognizes the intruder as Mitsuko Souma. Megumi briefly considers attacking Mitsuko but hesitates. In the novel and manga, the teacher deliberately calls her phone again, alerting Mitsuko to her hiding spot, while in the manga Megumi crushes the phone herself but still draws attention. Mitsuko approaches Megumi with feigned tears, claiming she is also scared and wants to team up. Believing Mitsuko's act, Megumi embraces her, only to have her throat slit by Mitsuko's kama. In the film, a similar deception occurs: Mitsuko enters, pretends to be friendly, laughs at Megumi's stun gun, and then kills her with the kama after a brief moment of trust. Megumi is the fifth student to die in the manga and the first victim of Mitsuko in all versions.

Throughout the story, Megumi's role is that of an early casualty whose death underscores the brutal, deceptive nature of the program. She has no special combat abilities or skills; her only weapon is a diving knife in the novel and manga or a stun gun in the film, neither of which she manages to use. Her motivations are purely survival and a desperate hope to escape through outside contact. Her key relationships include her friendship with Noriko or Mizuho and Kaori, her crush on Shuya or Shinji, and the antagonistic dynamic with Hirono and Mitsuko. She experiences no character development because she dies very early in the game. Her most notable trait is her excessive trust, which makes her an easy target for Mitsuko’s manipulation. Megumi Eto serves as a tragic example of a normal, kindhearted student who is wholly unprepared for the violence and betrayal demanded by the Battle Royale program.