Mark is, first and foremost, someone with a deeply peaceful and gentle mindset. He enjoys nature and serenity, doesn't know the meaning to the word 'confrontation', and takes the time to find the inner beauty in all things. He has a fondness for drawing things as he sees them in his sketchbook, regardless of what they are - from blades of grass to Muggle lawnmowers - and strives to make them look beautiful in his own definition of the word.
He can be seen as quite quirky, as his attention is easily caught by a variety of things - he can't sit still for too long, various outer stimuli striking his fancy, and as such he needs to investigate them. A curious person by nature, this makes it so he knows a whole lot about not much at all, and he's a treasure trove of useless trivia. It's commonly said that, even if he doesn't know what the hell is doing, he knows a thing or two about the activity that most others present don't.
The man is quite naive, having been brought up in an environment where everyone cared for him - he's very permissive of other people's faults, sometimes to his own detriment. He doesn't seem to care about where people came from or what they did in the past, opting to judge them on their actions rather than hearsay. He feels like he's compelled to do so due to his own social awkwardness - the more permissive he is, the more permissive others will be of him, hopefully.
While not wishing any harm upon anyone, he can certainly stand up for himself and others. He can't stand prejudiced people, and will actively become a roadblock for them should he spot them. If it means drawing out his wand, then so be it.
He actually rather enjoys social gatherings, despite his awkwardness - he enjoys to watch people talk and see how to behave around each other, both due to simple curiosity and to know how to improve his own mannerisms. He also finds humans (and fellow sentient beings) fascinating creatures, and wishes to know more about them in general - sometimes, this curiosity can be perceived as a bit creepy.