Bradley. (spoiler!)
It's a pretty shame that no one is talked about her death. I personally think she was such a good dark character and I loved how she killed her dad's enemy.
shareI was thinking of that last night. What I think is interesting/sad about her character is that her death is the one that remains most mysterious/unknown. All of Norman's other kills are attributed to him, or he was at the very least suspected, like with Miss Watson (and I'm sure that after the fact, after the series ends, background characters would give that more attention).
But Bradley is assumed to die by her own hand way earlier than she did. And if they ever found her body, first it'd be a job just to ID her, given that it wasn't legally her car, and she'd be in an advanced state of decomposition and so on. After they ID'd her, it would probably just be assumed that her father's enemies killed her and made up the fake suicide. At the point she was found I don't know if they'd be able to tell the discrepancy in time of when she supposedly died v when she actually died, and they might not even bother to check.
The only person left who would have any idea of what really happened to her would be Dylan, and that's only if he'd bother to really think about it. I doubt that, as he'd want to just put everything behind him. And I doubt he'd really want to bring it up to Emma "Oh by the way I think maybe Norman had another kill," what would be the point? At most I can see him maybe some night twenty years down the road having a bit too much to drink and then bringing it up, but even then. Dylan and Bradley got along, but Norma, Caleb, Romero, Norman had many victims that Dylan was much closer to, so she'd be like an afterthought to him probably.