Galka subscribes to the battle manga school of thought.
I don't think this Karin is a yandere though. We're seeing one trait and jumping the fence. This one is confident, selfish and narcissistic. We see it through her choices. Idol? All eyes on her. School fest doing a Cafe? Literally said "perfect all eyes on her" she's all about ME ME ME and only me and loves that part of the MC that depends on her the most. Gives her that fix.
It's frightening in a way, how nice that feeling if being relied on can be