Mark's route is quite different from others. Looking back at Mark's Episode (side story) even shows that Mark was the first person to see Walter back at the train station since day 1 of his arrival.
If you have not completed his side story, I suggest you would go and read it right now.
I don't want to spoil anything my friend.
There's a connection for sure between Walter, Halberd, Mark and his brother.
For now I cant seem to collide every pieces together yet, soon the story will be unfold.
I'm guessing that Mark will revenge for his brother. Probably things will get heated up as the storyline progress...