Mountjoy comes from Bishop's alternate future. He was taught by the Emplates to possess the bodies of others. He came back in time through Trevor Fitzroy's while possessing Bantam. He left Bantam afterwards, but hunted him down once Fitzroy was dead, because Bantam was the only person left who knew of Moutjoy's existence.
While hunting Bantam, he fought and defeated Bishop by possessing Storm and using her as a hostage. Mountjoy escaped and Bishop tracked him to a NYPD station. Mountjoy absorbed Bishop and used his body to get into the X-Mansion and attack the X-Men. He absorbed Archangel, Gambit, and Psylocke, but Bishop used a hologram of his sister Shard to lure Mountjoy into the Danger Room, where Mountjoy tried to absorb the hologram. While Mountjoy was confused, Bishop blasted him forcing him to release the three X-Men.
Mountjoy was able to merge with others and absorb their memories. While merged with a mutant, he can use their powers at will. Once merged, he could choose to ride along within his victim and subtly control their actions, absorb their body into himself and use their powers as his own, or simply separate himself from them.
He also projected a "Basilisk field" which slowed his opponents' reaction times within a five-yard radius.
To describe his bodily-possession abilities, Mountjoy would use business terminology including "silent partner", "hostile takeover", and "divest".