A few theories:
Alternate/future Uchi:
Wants to save Yotusha from having to save him so many times. Only problem with this theory is that she has saved him before, she is just trying to get him the most happy he can be. Why not just tell her the consequences instead of trying to get him to commit? Also, how can he be time traveling?
Father:
Feels a little like a red herring and, similar to the guy in the flashbacks, it feels to obvious. Plus, why would her father, who might not even know she exists, care about some random kid she met at school as a young child?
Brother:
Most likely, but still, why does he care either for Uchi or Yotusha. He didn’t know about Uchi until recently. It seems he is helping Yotusha for the benefit of having someone who can time travel.
Hacker-close friend (I forgot her name):
We haven’t seen her a lot and she knows Uchi and Yotusha. Maybe she secretly loves Yotusha and wants to kill Uchi? However, if that is the case why risk her life with people like her stalker/obsessed classmate?
One of the bullies/participants:
Maybe one of them (my money on any of the bullies is on the background bullies from the subway) is doing it out of sick pleasure and enjoying controlling multiple people.
Nobody we have seen:
It is possible that some sicko is just f-ing around with Uchi and finds sick joy from it. With Yotusha acting as a new fun toy. It is also possible that it is an AI that misunderstood some order or is following one someone made but thought it failed and gave up on it.