It's probably most likely his parents or sister

I was thinking stalker

Maybe but, for him to react like that it's possible that his parents/sister contacted him, I mean we didn't hear about any of them since he became a streamer and his parents had high expectations for him so imagine how they reacted to that TT

But her sister was very supportive, and toxic japanese parents in other mangas always go no contact with the children that aren't perfect.