Legion is a character from Fire Emblem: New Mystery of the Emblem ~Heroes of Light and Shadow~. He also appears as a SpotPass Einherjar in Fire Emblem: Awakening.
New Mystery of the Emblem ~Heroes of Light and Shadow~
Legion first appears in the prologue of Fire Emblem: New Mystery of the Emblem ~Heroes of Light and Shadow~ , goading some brigands into attacking a small Altean village. As this happens, the 07th Platoon led by the Hero of Shadow and Katarina appears to defeat them. Beaten, Legion smirks at his enemies as he makes a shadowy escape.
A short while later, Legion attacks Altea Castle while Prince Marth is holding a knighting ceremony for his new recruits. Acting as a diversion, he allows his partner Katarina to fight against the 07th Platoon, as he deals with the main army. When Katarina was defeated in battle, he saved her from death, and calmly told her that there's always another chance to kill Marth.
After Marth and his allied defeated General Lang in battle, and were declared traitors by Emperor Hardin, Legion hired a band of mercenaries to kill the prince. Despite this Altean spies had spotted his ambush, and Prince Marth was able to escape death once again.
When Legion returned to his hideout, his "mother" Eremiah gave him a scolding for failing too many times. He vows to finish the job once and for all, and took all of his brothers to a castle in the Desert of Death, where he captured General Horace, and used him as a lure. At this castle, Legion reveals that all of his brothers retain the same consciousness, and only by killing the main Legion, can Marth actually defeat him. After a hard fought battle, Marth found the elder Legion and finally defeated him for good. When his brothers returned home, Eremiah ordered their execution for failing, and Legion was finished for good.
In a free DLC episode released for the game, it's revealed that Legion and his brothers helped his fellow assassins Katarina and Kleine defeat the remnants of the Soothsire Bandits after the events of Fire Emblem: Shadow Dragon. After defeating the bandits, Gharnef gave his permission for Legion, Kleine, and Katarina to assassinate Marth.
In Fire Emblem: Awakening, Legion appears as a SpotPass Einherjar. Like in the previous game, Legion is a berserker, and he plays a minor role in the Lost Bloodlines DLC story arc.
In Awakening, Legions team is composed of Berserkers with the exact same stats and skills. This is a reference to "legion" of brothers.