Like Nathan early in the series, Mark is a coward, however he always run from the predators even if someone is being held hostage. This may have been, since he only lived with his grandmother and doesn't have a father-figure to look up to. Mark is also envious of anyone having something good that he doesn't have, like when another student has some cool lucky charm.
But despite his combination of jealousy and cowardice, Mark has a good heart when he decided to buy Nathan lunch or give him tries at games out of guilt for taking the beating. At one time, Mark rescued another student from falling off the third floor of a building. He also went against his policies to save a child, only because he doesn't want to see him die.
Mark was Nathan's only friend between the time Winona was abducted by a swarm of beasts and when the storyline began. However, Mark was manipulating Nathan in order to use him as a scapegoat for time to get away from bullies. He did help out Nathan through school by being his tutor and helped pickpocket Nathan's stuff from bullies after they stole it.
Warrior's Awakening Arc
After Nathan gotten beaten up and his goggles (actually his Trace Visor) was thrown in the tunnel by bullies, Mark emerged from a rock. He tried convincing Nathan not to go after his father's goggles, but Nathan jumped down for it.
Nathan returned the next morning which relieved Mark and was about to see Nathan's Energy Gauntlet from his backpack, but was taken away from Dr. Brimstone. Mark managed to steal the keys for Nathan to get his backpack, but Mark fled from some rat monsters.
Mark grew jealous of Nathan having Serena Thorne and Dimitri Stone being his friends. When Professor Wilder attacked Rebel City with some monsters, Mark saw Nathan, Serena, and Dimitri fighting off the monsters. But Mark was captured by Aviva to be an experiment of the Monster Empire along the few citizens captured by Wilder.
After Nathan had defeated Wilder, he had found out that Mark and the other captives were transported to another Monster Empire Lab.
Mark returned to Rebel City seemingly normal and was being introduced to Winona and Wolfe Savage. Mark didn't trust Winona for some unknown, when Winona remembered kidnapping him. Unfortunately, Mark had been part of an intangibility experiment and was being mind controlled to fight Nathan. Mark was defeated when Nathan used pepper to have Mark lose his concentration and get knocked out. Winona showed up and somehow released the mind-control on Mark, but he was quickly captured by the student council (the mysterious students that sent Mark here). Nathan and Winona were threatened that if they want Mark back, they would work for them, but had four days to decide.
When Lucas Rush was inside a cell around Mark, Nathan and the others agreed to work for the student council by helping them out with the project. After being freed, Nathan and the others found out that the project was used to transform Rebel City into a flying city to keep away the Monster Empire or GEM Military.
Mark was wondering around some desert (somehow free from brainwashing and escaped from Rebel) to find Nathan and company. He wandered into a town that was taken over by some bird demons. Mark tried to stay hidden and escape until he found Nathan, but without his memory and heavily marked from his battle at Mighty Mountain.
Powers and Abilities
Ever since Mark had gotten experimented on by the Monster Empire, he can become tangible to escape from any danger. He also uses the intangibility to steal stuff from people, like when he was brainwashed, he stole Nathan's house keys.
Throughout the time of running from bullies, Mark always managed to escape from them while Nathan usually gets beaten up by them. Even without his intangibility and as long as Mark is alert, he usually finds a way to escape from any dangerous situation which can help others get out of.