'M was a Glitch Pokemon, and the main villain of the Unova arc.
'M began life as a project of Cipher's known as the Glitch Project. Four years prior to the Sinnoh Arc, a teenaged Tagg returned to Orre and met Professor Krane at his lab where he was informed of it, but unfortunately neither learned much due to most of the data being corrupted. It was active even before then, as it managed to corrupt both Steven Stone and Wolf, the latter forced to become a member of Steven's private army.
'M first came to the proto J-Team's attention when it turned out that it was possessing Steven Stone and had him fight some J-Teamers in Undella, later possessed Clay's Excadrill and had it attack Virizion, and also mutated Iris into a half-human, half-Haxorus Glitch monster. It also took the children of a Day-Care and turned them all into shiny Charizard 'M, removing the children's vocal cords because they found their screams of distress annoying. At one point, the proto J-Team also fought 'M possessing a 50-foot tall Clay.
Its final possession act was to possess Jane and her Pokémon, and have her trap the proto J-Team inside the Dream World as they were attempting to wake up the Elite Four members who were stuck in a Glitch induced coma. During the ensuing three way between them, 'M, and Ghetsis' Team Plasma remnant, Jane was unable to break 'M's hold despite Anom's attempts through their mental bond, and was accidently killed by Daisy's Zoroark strangling them, and the Glitch was more directly responsible for Wolf's death by having an 'M-possessed Eliza shoot his Swoobat out of the air with a Shadow Ball, the resulting fall killing him.
After Arceus revived them, the proto J-Team fought 'M once again in the ruins of N's Castle, managing to destroy its data core and free Alder and the transfigured Charizard 'M from its control. However, the bio core managed to escape, and they were forced to travel to the Grey Ruins to find it, 'M having turned the area into what could only be described as being made of viscera. After making it to the core's location, the Glitch sent them to Glitch City, where the final battle began.
The fighting continued until Anom sacrificed himself by allowing 'M to absorb him, which inadvertently removed its anti-legendary barrier, and gave the legendaries the opening they needed to destroy it.
Unlike Missingno, who towards the end was a more tragic figure, 'M was a more sadistic being that regularly reveled in the suffering of others.
Powers and Abilities
'M's capabilities were similar to Missingno's, being a techno-organic monstrosity capable of warping reality around it and possessing others, though save for its anti-legendary barrier it was technically weaker than the previous Glitch.