Marcus is a student at Mission Creek High School. He doesn't like Leo and Leo does not like him. In Concert in a Can, he, Chase, and Adam got in a band, he broke his guitar and tried to pin his broken guitar on Leo. In Mission Space he found out about the lab, and put security cameras in the lab, for him and his father, to spy on the Davenports and the Lab Rats.
In [Showdown], we find out that Marcus is an android, and that he might not live up to be 16 years old. He was crushed when chunks of cement fell on him, but his arm was free, so we aren't sure if he is dead, or still alive.