Hina

Blending in is the point. Hina keeps to flat, forgettable clothes and draws no second looks, her one memorable feature being a brown bob whose bangs hang far enough to bury her brown eyes.

That changes the moment she needs a mark to trust her. Out come a cuter outfit and heavier makeup, the bangs lift, and the eyes are suddenly on display.

Ordinarily she is timid, awkward with strangers and worse with men, and Rin Yaritezawa is the one person she genuinely warms to. The switch flips the instant any man shows Rin interest. Jealousy that extreme, layered over open contempt for men as a species, has her filing all of them under beasts, and protecting Rin justifies anything, up to dosing a guy with Trigger for the crime of making conversation.

Number 6 supplies her at some point with a reworked Trigger that needs only to be swallowed. Armed with it, she spends weeks attending mixers alongside Rin and spiking the drinks of any boy who circles her, producing a string of Instant Villain incidents. Police notice, but suspicion falls on Rin, who organizes the events, and Midnight is assigned to shadow her.

Rin's next mixer is booked with the Mad Hatters, though Koichi Haimawari turns up in place of the band's leader. Midnight, having crossed paths with Kazuho Haneyama tailing him, has the Midnight Boys peel away the two other girls due to attend so that she and Kazuho can slip in as substitutes. Things stay quiet until Rin throws up and Koichi shows concern, which starts her falling for him and sets Hina off. After a costume change and fresh makeup, she doses drink after drink for Koichi, and he declines every one, leaving other customers to drink them instead.

Those customers' Quirks go haywire on schedule. Koichi's efforts to contain the chaos only make him more attractive to Rin, and Hina finally tries to force the drink on him, at which point Midnight uses Somnambulist to drop everyone but Kazuho, Rin and Hina. Confronted, Hina admits to all of it and explains that she cannot stand watching Rin with boys. Midnight sympathizes, puts her to sleep anyway, and arrests her. Questioned the next morning about the Trigger, Hina names her supplier, a dealer whose Quirk grants acceleration, and investigators tie him to the destroyed evidence from the Osaka Incident.