Zangetsu

Zangetsu is a character from Bleach, is Ichigo's teacher when it comes to power but, he also has many forms which are strong and come in handy when Ichigo needs power.

May just look like an old man but he's quite strong and wise, and thats how Ichigo win his battles by lending him strength

But Zangetsu has more than just that form

His younger state seems to me that he's stronger than when older

Thats how Ichigo approached the first time like that

And strikes at him, because he doesn't want to teach Ichigo the final getsuga tenshou

Hollow Ichigo is the negative side to Ichigo, which he also is zangetsu in a way

But when zagetsu summons him to fight Ichigo he appears in vasto lorde form, and together fuse up as Hollow Zangetsu

This is when he really wants Ichigo to not learn the technique

But thanks to Zangetsu, Ichigo had an evolution

Thanks to Ichigo's source of power he is now the strongest, but the show should've been about Zangetsu, no?

GETSUGA TENSHOU!