There is pretty much no information regarding Anonymous Shinobi, as he lives in the shadows (as in, actually lives on the dark underbelly of society, not does evil things and hides because of his tragic past until somebody comes along to prove his ways wrong). He, like any actual ninja worth hiring, has concealed virtually every aspect of his past, combat abilities, and even his existence itself. The only reason we know he exists is because the lords and crime bosses he has been (presumably) hired to assassinate obviously didn't poison or stab themselves.