Maglor is one of the seven sons of Fëanor. He is the second son, originally famed for his musical ability. While he was a party to the many atrocities committed by the Feanorians, he was often more sympathetic to the victims than his brothers. He was also the only one of the seven who had no recorded death. While the others were killed in battle or, in the case of Maedhros, committed suicide, Maglor simply wandered out of the history of Middle-earth, supposedly searching for the Silmaril he threw into the Sea after it burned his hand.
It is these qualities, perhaps, that make him a Sue-target, when he isn't being slashed with one or another of his family members.
On one memorable occasion, a Sue created a doppelganger of Maglor through bad phrasing, thus creating one of the PPC's most famous (or infamous) agents, Dafydd Illian. More recently, a character replacement of Maglor was recruited, ending up as Agent Hawthorne.