Mark is introduced in the beginning of Series 5, and throughout which series he becomes a supporting character. Mark was a young Scottish man living in Wertham when The Storm hit, giving him the power of being permanently trapped in the body and shell of a tortoise.
Nothing is revealed of Mark before The Storm, although from that point on he was stuck in the shell of a tortoise crawling around the estate. Mark is first seen during the opening scenes of Series 5, as a tortoise attentively looking at the flyer of a support group for people with powers before the flyer is blown away by wind; the tortoise then begins scuttling off to presumably find it.
Mark as a tortoise is later frequently seen on one of the chairs at the Power Support Group when Rudy Two beings attending, after the tortoise evidently found it himself. Although a tortoise, Mark was able to communicate with the rest of them, and explain his name and plight to them; however how this was done or what form of communication this took is never seen or explained.
When Abby goes to the Power Support Group with Rudy and Finn, Abby showed an instant a fascination with Mark (unusual since Abby rarely showed enthusiasm or emotion for anything) and absconded with the tortoise. Mark is not seen again at the Power Support Group, but Abby is seen reading books like "How to Love your Turtle" and Mark ends up making his new comfortable home in Abby's locker at the Community Centre. At one point she is seen talking about taking Mark to the cinemas, because he is a "big Tom Hanks fan". Abby then later declares her undying romantic love for Mark to Rudy, Finn, Jess and Alex, and resolves to finding a way to make Mark human again so that they can be together.
Abbey knows that getting Alex to have sex with the tortoise would turn it back human through Alex's Sexual Power Removal. Abby tries repeatedly to get Alex to have sex with the tortoise to remove his power but he refuses, he later tries to but cannot bring himself to do it. Alex, when later under a gypsy curse that forced him to help people when asked, decided to get drunk on vodka to do the deed, with the tortoise also getting drunk. They were about to go through with it, when distracted by the after-effects of the power of Lucas - that is, a failed suicide attempt by Finn. Alex held Finn up from hanging himself, whilst Mark scuttled off to get help, both of them together saving Finn's life.
At the one year anniversary of The Storm / Abby and Rudy Twos' birthday party, Abby discovers that ecstasy reverses powers and decides to give the tortoise a pill, which consequently reverses his power and finally turns him back into a human. Mark tells Abby that he misses her when she is gone and that his is afraid to return to being a tortoise so she plans to 'get a big bag of pills and get Alex to shag [him]'. They kiss passionately and being a relationship, albeit briefly since they are warned by Alex about the dangerous situation regarding Sarah.
When trying to evade a possessed Sarah with Abby, Abby tells Mark to cover his eyes from Sarah's Hypnotic Breasts, which he does. When they bump into her, Abby tells Sarah that her powers will not work on her since she too is a girl. Sarah then turns Mark inside out through her other recently acquired power of Object Inversion and Mark dies verily bloodily. Abby, splattered with Mark's blood, dumbfounded and in shock, is then freely Satanically converted by Sarah.
Mark's power was that he was permanently trapped in the body of a tortoise. It is unknown why Mark got this power, since nothing is revealed about him before The Storm, although it may be because he "lived in a shell" in a symbolic state which became literal. Despite being in the body and shell of the animal, Mark retained his memories, personality and consciousness within it, since the tortoise shows evidence of intelligence, and when made human again can remembers things that happened to it as a turtle. Mark was clearly able to communicate with others around him, such as with Abby, Alex, or others at the Power Support Group, in telling them his name and his plight. Exactly how this was done is not explained since Mark cannot speak as a tortoise; although a slow visual communication would be more logical, some form of telepathy is implied/hinted at.
"I get so lonely when I'm in your locker, and you're out on the estate. I'm just in my shell, waiting for you. What about when the drugs wear off?" - Mark to Abby during his brief stint as a human again, due to drug-induced Power reversal.