He brainwashed Wolverine
to make him loyal
Details of the Secret:
When Wolverine first joined the X-Men, Xavier operated on Wolverine’s brain telepathically. Although removing the mental blocks and controls installed by Romulus was done with Wolverine’s consent, Xavier went on to remove some of Wolverine’s memories – notably those involving Romulus – and to instill in him a loyalty to the X-Men (and by implication, to Xavier himself) that he did not feel before then.
Kept from: Giant-Size X-Men #1, May 1975.
Who knew the secret: Xavier and Moira McTaggert. It’s likely that Romulus worked it out, too.
Secret revealed: Wolverine: Origins #29, December 2008.
Notes: Unusually, when this secret was revealed to readers, it was unknown to Xavier and known to Wolverine – Xavier having recently lost many of his memories and Wolverine having regained all of his. Technically, it was revealed to Wolverine in House of M #8, January 2006, in which he regained all of his memories, but he did not share this information for while.