he's the manifestation of Ichigo's Quincy powers, and the "inner Hollow" White Ichigo is actually the spirit of his Zanpakuto. It makes complete sense that "Zangetsu" would have this power.

It appears on the same side of Ichigo's head as the horn Masaki could not destroy from White—quietly implying that some of White's essence remained within him. Ichigo's Horn of Salvation ability ...

It's thanks to Isshin that Ichigo had gotten a powerful Shinigami side in his odd genetics. Near the end of the series, it’s revealed that White is the manifested spirit of Zangetsu the ...