Martel is one of the Chimeras that joined Greed to get revenge on the military for what was done to them in the 5th Laboratory.

Spoiler

She gets inside Alphonse's suit to make him unable to move when he gets captured by the Chimeras. She ends up developing a relationship with Alphonse, who tries to understand her motives unlike Ed. She is killed by Wrath right after taking shelter in his suit as she had done many times.