Remember that this is a revenge story, for someone to try and kill uichi as a revenge you need either:
- something uichi to be blamed as a kid
- something uichi to be blamed as a teen or adult + a time traveler
The first option fits hacker girl (jealousy) except for three things, first she was the one who gave yotsuha the info about the sister talking to clover, also yotsuha went away on her own, there's no reason to be jealous of uichi if he's not by her side, and third, she has no reason to call herself clover, as a bonus... If she was jealous and psychopathic about it, she wouldn't have used yae sin he would technically be another contender for yotsuha.
Now the time traveler theory makes much more sense since yotsuha either stays with uichi or dies trying to save him, that's actually revenge worthy, and now the suspects can be many, including hacker girl again but older, the brother, or a new player since in the case ofnyotsuha dying, any of them could've found the original clover and took it as their symbol