This contains spoilers up to chapter 112 (not much of a spoiler, but still).
White Ichigo (a.k.a. Hollow Ichigo) appeared in the Hollowfication process, when Urahara had cruelly thrown Ichigo into a pit, and he is the Hollow part of Ichigo's soul (and power).
Now, take a look at this (this is from chapter 112, right after
White Ichigo first appears and gets beaten by Ichigo):
It shows not only that Zangetsu is acquainted with White Ichigo, but also is in apparently well relations with him ("sorry to have bothered you"), and is also apparently able to summon and unsummon him ("send me back already").
How and why is Zangetsu (a Zanpakutō!) able to summon the Hollow inside of Ichigo? Shouldn't a Zanpakutō spirit and a Hollow Spirit be natural enemies?