It's a show a saw a few years ago and can't remember the title.
I say "few" years, but considering how long time flies may be it was like 10 years ago.

It was your run of the mill forgettable urban fantasy anime.

Characters fused with heroes/spirits to fight monsters.

MC was a dude that managed to save the world once but lost his partner on the process and joins an organization with several girls

There was in the same organization a dude that was set as the "rival" and that everyone treated with respect. But there was a "joke" twist where they reveal the rival was actually just chuuni and din't have even an spirit partner.

I'd like to find the title since everyone except that last thing was quite generic so I can't find it