Mark Dimond was one of the acolytes of Bobby Pendragon on Second Earth. He is known as a nerd, who reads comics, eats carrots to improve his eyesight (he wears glasses), and his mom buys his clothes. He and Bobby still listen to 80's music. No one messes with Mark if he is with Bobby. Mark hangs onto Bobby like a lifeline. Otherwise kids will pick on him. He and Courtney Chetwynde helped Bobby defeat Saint Dane on Eelong, by going there, and bringing the antidote to a poison Saint Dane brought, to destroy the food chain. Where he and Andy Mitchell (who was actually Saint Dane in disguise) got Forge from Third Earth and took it to Second Earth where it was used to help Mark and Andy/Saint Dane win a science project and was later brought to First Earth. It was then sold to KEM, thus changing the timeline for Earth technologies. He's also the father of the dados on Quillan, and the 3 Earth territories.
In Raven Rise, he gets pushed into the flume with a large group of innocent people and gets exiled to Cloral. While he and a group of the exiles are on Cloral, he is reported to protect them during their time as slaves of The Raiders. Mark and a group of exiles escape and travel to the flume to wait for help. Mark eventually becomes tired of waiting, and transports the exiles with him to Third Earth, where they join the resistance in New York.