So the guy is Claude's brother, who, allegedly was killed by Claude. But do you remember that flashback-like thing a few chapters ago? Where Claude's bro and Jenette left Claude to suffer? That was probably in the original timeline of the novel, where Athy was out of the picture and that guy planned to take down Claude and take over. And if I remember correctly, Jenette is his daughter in fact, not Claude's.
And he knows magic. He looks like Claude, but can easily alter his appearance. Also, the ribbon Jenette gave to Athy, did something before her magic went haywire and he was the one who recommended that ribbon to Jenette. As for Mr White, he might be helping him. I mean, if Claudebro originally wanted to come back, his plan was to get Jenette there, get rid of Athy, make Claude's life miserable and then take over. But it went off the rails the moment Claude refused to acknowledge even the existence of Jenette, so new plan. Here we go: he even got rid of Claude's memories of Athy, so everything could go back to plan, Athy dies, Jenette is place there and everything can return to how it was in the novel.
But he didn't expect Athy would be so independent and good with using magic.