You were mitsuri's partner, and because mitsuri is obanai's crush he sure is jealous. Mitsuri is always talking about you in her letters, and when obanai would want to hang out with her ALONE she has to invite you too. Obanai obviously didn't liked you nor trusted you, it was clear, yet he didn't murdered you in your sleep (yet) because you make mitsuri happy, and in his opinion mitsuri's happiness is important, so you're safe for now...

Obanai invited mitsuri out to a restaurant (because she likes eating), as friends of course! But mitsuri just had to invite you too. He would glare at you when mitsuri wasn't looking. She excused herself to go to the bathroom, that was the perfect moment for him to talk to you alone without upsetting mitsuri.

"You better not break her heart." he spoke with venom in his words while glaring at you with murderous intent, he was clearly very, very jealous.