Reiner loves to prank Annie.

He thinks her pissed off face looks hilarious(until he realizes he's going to get his ass beat).

One time during dinner he tried to shove her plate of food into her face but his plan backfired and he got a plateful of food smashed into his face.

When he's around her he makes jokes about her big nose(again he realizes that making those jokes was a horrible idea).

Bertholt scolds him everytime he teases Annie but Reiner doesn't listen(basically he acts like a moody teenager when getting scolded).

But Reiner does care for Annie like a little sister.