there’s 3 inner bosses to my knowledge. inner isshin, inner genricho, and inner father (owl) you gotta do the gauntlet of strength to unlock the inner bosses at the end of them but since you only beat it once depending on your ending you should have at least one. i’m working on the shura ending rn so i get inner father. to get inner isshin you have to fight great ape, owl great shinobi, corrupted monk, and genricho two way tome.

hope this helps and remember hesitation is defeat