my bet is on the girl. she had henna tattoo, which could fade in time or be rubbed off. also, she was only drowned and unlike the other guy, she wasn't seriously harmed. simply said, she got off easy. too easy, in fact. she also didn't have much alibi, and no one else to back her words up. and the fact that she deliberately warned the mc not to investigate into the case further was too suspicious
I also still have some suspicions about her! Let's see...she said she was attacked because she was investigating, but who knew about that besides her? She overheard the conversation between Tokiwa and Hinata in the hospital room, but never told anyone about that. Also, coming earlier to the meeting to check the hose shouldn't be suspicious. I see no reason for a possible culprit aim her. Together with what you raised, this makes her account at least suspicious.
There many other mysteries though, CAN'T WAIT for the next chapters!!!
why is it tagged as shoujo