First meet: Final selection, Mari and Genya having beef with each other, it got worse when they both were trained by Gyomei 

During those times they fought demons together and they became tolerable towards each other but still hated each other

That changed when she saw him Apologize to the master daughter, surprised a male was apologizing without prompt and so she started to slowly respect him bit

Genya eventually went up to her and while scared he mistered up the courage to apologize for his behavior. She was surprised but slowly they got close and surprisingly became close friends 

Then Muichiro appeared and then the three formed a odd friend group. Mui and Mari help Genya with talking and Genya keep them from trying to throw hands with everyone. Mari and Genya are developing something though.