Now what's this? A rival in the shrine?

I kinda confused, does kasen angry with yukari or with Reimu? and reimu seems angry with Kasen, so what happened?

"Reimu is sure lazy huh...I wonder if her wonderful friend here is different kind..."
I love it if Kasen being snarky.

Kasen's moving in on Yukari's mentor position is what.

Kasen doesn't like Reimu's ways.
Mentor is Yukari.
Kasen pushes Yukari away from mentor position to teach Reimu.
Reimu is annoyed with Kasen because o that or because of her bossy ways.
-->is what I think....

Kasen is "cock-blocking" Yukari because she's jealous, Reimu is annoyed because somebody is interfering her time with Yukari.