Martel is a member of the gang led by the Homunculus Greed. She's a human chimera combined with a snake.
Even though Martel knows she's suppossed to be holding people hostage, she still enjoys having conversations with them and teasing her teammates, as seen with Alphonse when she and the other chimeras kidnap him. Although even though she teases her teammates and acts like she hates them, she has a deep sense of loyalty and will often act on impulse to save them.
Her unfortunet death was swift when Al tried to escape with her in his armor. She was crying madly about her dead teammates and wanted to make Al go back to avenge them. However, the ran into Bradley who killed her inside Al's armor, leaving only a bloody mess and splayed limbs behind; R.I.P. Martel.