Honestly I've been trying to find a clear answer on this for a while and it's a bit of a mess. 'Do It Oppa' isn't one single, massively famous manga, which makes pinning down the definitive cast tricky. From what I've gathered from scattered raws and forums, it seems to center on a female lead, often a college student or office worker, who gets tangled up with an older guy she calls 'oppa'—but the dynamic is less sweet and more... intense, maybe even manipulative? There's usually a best friend character who serves as a warning or confidante, and sometimes a rival love interest who's a bit more stable. The art style in the panels I've seen leans heavily into the smoldering gaze trope for the male lead. It's one of those stories where you're never quite sure if you're rooting for the couple or desperately hoping the FL runs for the hills.
I think the ambiguity is part of the draw, honestly. You go in for the 'oppa' fantasy and stay for the psychological tension. The characters aren't deeply fleshed-out archetypes so much as vehicles for a specific, moody romantic thriller vibe. It's less about who they are by name and more about the push-pull power dynamic they represent.