Mesmero is a mutant who served Apocalypse.
Powers and Abilities
Mesmero is a telepathic mutant, allowing him to read and control minds, scan and manipulate memories, and project his thoughts. He is capable of manipulating multiple minds at once, as seen in the episode Mindbender when he placed Jean Grey, Shadowcat, Spyke, and Nightcrawler under his control. When his mind was temporarily linked with Apocalypse, his powers were greatly-enhanced, allowing him to defeat Professor Xavier in a telepathic battle.
However, after Apocalypse was freed, his powers were likely decreased to the point where they were inferior to both Xavier and Jean's. This seems to imply that the bulk of his telepathy came from Apocalypse himself, and that his own abilities, if he had any at all, were weak.
Season 2
In the season 2 episode Mindbender, Mesmero takes control of Jean Grey, Spyke, Shadowcat and Nightcrawler, and uses them to steal all the components he needs to assemble "The Key" to unlock the first of three doors that seal Apocalypse within his tomb.
Season 3
Mind-controlled Gambit to help steal an object that would, when combined with the second object create a Mystical Arachnid that guards the 2nd door and needs to be destroyed before it can be opened (Under Lock and Key).
For the last door, Mesmero mind-controls Rogue into taking everyone's powers, and then has Mystique open the last door from inside by placing her hand on a Mystical Stand that turns her into stone.
Season 4
Ascension I
Mesmero is ambushed and captured by Wolverine and Jean, the latter of whom psionically searches his mind, then stating that Apocalypse appears to have abandoned him. Mesmero protests, but Professor X asks Mesmero for cooperation, warning him that he can and will seize the information from his mind by force if necessary. Mesmero begins to reveal what Apocalypse is planning, but then stops, stating he has said too much.
Professor X then enters Mesmero's mind, learning about Apocalypse's plans for himself. With that information, Charles and Storm leave for Egypt, leaving Mesmero captive. As this is his last appearance, Mesmero's ultimate fate is unknown.
Trivia
Mesmero is given Mr. Sinister's role in this universe as Apocalypse's aid.