Agent X is a contract assassin in "Agent X".
She (Agent X) is very good at her hand to hand combat skills, and is very good at her attack skills and fooling people.
Agent X's past is a complete mystery. She had never failed a mission and is hired by the Grand Master to retrieve a microchip from M.I.9 agent Oscar Cole. In her first appearance, she appears as a silhouetted man in a hat, with a robotic voice. This is an edited computer image to keep Agent X's identity hidden. Agent X tracks Oscar Cole from his old home, to a safehouse, to an M.I.9 training facility, before settling in St Hope's. At each stage, she demolishes all opposition and leaves an agent behind to eliminate anyone who tries to track her down.
Her true identity is revealed at the end of the episode: She is an eleven year old girl named Della, who passes off as shy but alarmingly intelligent and a computer genius. She finally manages to corner Oscar and easily defeats him in unarmed combat. Oscar tries to bargain for his life, resorting to the sort of things that girls her age would be used to. He finally asks her what her favorite toy is, and she stops attacking. It is revealed that she, like Oscar, grew up unhappily as she had a spy as a parent too (a horrible one), who did not give her any toy's. She joins M.I.9 eventually and loses herself among the several toys Oscar gives her.
Agent X is an astonishingly intelligent bounty hunter. She is cynical, strong, dangerous, violent, unstable, highly cunning and ruthless. It is revealed that she is still a child in an adult world and has typical weaknesses of a child, such as fear of neglect.
Nevertheless, she is incredibly skilled in unarmed combat and has an extraordinary level of physical strength, capable of effortlessly defeating a group of M.I.9 operatives. She is resourceful enough to keep her identity concealed completely to an extent where nobody even knows what she looks like.
Her young appearance is alarming, and possibly a disguise that she probably got too used to.