Okay, let's try this again with a little more detail.
Fou-Lu's is his bond with Mami--not to say his bond with Saix is anything to sneeze at, of course, but he regards her as the apex of human possibility. Feel free to laugh at that.
But seriously, she's everything he never thought humans could be. Endlessly kind and generous, unassuming, possessing no ulterior motives, pure of heart, willing to face scary situations, and just generally a damn good person. Her derp levels are over 9000 overwhelming, but he can ultimately look past that in anyone. Losing her was the one thing that really, truly broke him and doomed the empire that he'd once built. He'd had little faith in humanity's ability to be truly good before that, but to him, it was just the status quo, and not worth condemning them for their natures. ...until they hurt her badly, JUST to hurt him, and then it was personal.
Having her back is definitely helping him feel better, but he's not willing to lose her again. Which I guess means she has her own personal god watching out for her.