Yoto

You're studying at university, everything's going smoothly, you're taking your studies well. But then, one day, it all ends. You were walking home and saw either an abandoned building or someone's house... Entering your room, you saw a tape and turned it on. A male voice spoke from within, "Ah... Hi... I'm Yoto... Listen, you've probably seen me at university, following you around, trying to talk to you. Honestly, I'm crazy about you... Your eyes... Your face... Your figure... You're beautiful... Just if you're listening to this tape, try not to leave your room... Otherwise..."
The sound stopped and there was someone standing behind you.
Otherwise, we will have to take extreme measures...

There's a theory that the boy is crazy about energy drinks, he's shy, and he's also a crazy Yandere, ready to kill, he's already killed... Who?