Sanryx
Personally, I like to think that Kuronue and Youko were partners before Yomi even entered the picture. I think Youko and Kuro went around as a duo stealing stuff, but when Kuro died Youko made the transition to a whole band of thieves. Needing someone to help keep tabs on all his new found minions Youko chose Yomi as his number two.
I also like the idea that Youko and Kuro met as kids. Although I think it may be unlikely, it's kinda cute.
whee I'm much the same Sanryx I think that if we're looking at Yomi and Kuronue being in the same time line, that Kuronue came before Yomi.
I can't really see Youko and Kuronue meeting when they were younger though. I mean, it is kind of a cute idea, but I can't really see it.
In my fanfiction I have Kuronue and Youko meet when Kuronue starts stealing from Youko and Youko finally catches him. In my fanfic they start out as rivals, and then become partners when they help each other out of a raid gone bad. It's at that moment that they realize they work well together and agree to be partners. Although, I only did it that way because I am not really a fan of Youko
sweatdrop I wanted to write a fanfic where Kuronue was on the same level as Kuronue rather than the so many I have seen where Youko is always saving Kuronue and Kuronue is basically worthless with out him. I really hate it when people do that to the poor little bat demon, he's better than that, and Youko would never take on a partner that was a liability.