Kuronue

Canon Bio
Literally nothing is known about Kuronue's backstory except that he was once partners in crime with Kurama at one point, and he died telling Kurama to run instead of going back to save him.

The canon timeline suggests that this must have been some time before Kurama formed the gang of bandits Yomi was part of. There is no reference of Kurama having had a partner or equal at that time, only Yomi as his highest-ranking inferior. After betraying Yomi, Kurama went to Reikai and stayed there for the remaining time before his near-death ~15 years before the start of the series, and Kuronue is stated to have died fleeing from a raid on a castle in Makai.