She really does get angry only when douki and senpai get close together. Maybe she doesn't want to be left out.
She wants to be the meat in a douki sandwich?
(It turns out the dude is dubbed Douki-kun - "douki", apparently means a peer relationship, akin to how sempai/kouhai is senior-junior. The main character and the fellow she has a crush on joined the company at the same time, so they are douki to each other, whereas the rival is kouhai to both of them, and they are both sempai to her.)
