Marcus is the novel's boy genius, and somewhat tragic figure, who has a hard time relating to other people. His pretty stupid attack on Sal starts a series of events that eventually lead to Sal's injury in the streets that day – and to Marcus' own death (Chapter 45). Marcus's character makes us seriously consider the consequences of senseless violence and helps us realize the value of empathy and compassion.
Marcus, egghead that he is, is also the character in the novel most closely associated with time and time travel. His conversation with Miranda in Chapter 14 is crucial to our understanding of how the novel thinks about time:
"Einstein says common sense is just habit of thought. It's how we're used to thinking about things, but a lot of the time it just gets in the way."
"In the way of what?"
"In the way of what's true." (14.62-64)
Marcus comments in this chapter let us know that we're going to have to abandoned our conventional understanding of time (that time is linear and moves in a straight line) in order to understand the events of the novel. In other words, it's time to embrace some paradoxes – and some Einstein. Marcus and Julia (who will, in their adult lives, marry) explain this theory in a later chapter. Marcus tells Miranda that time is like a snapshot – "It's like having a drawer full of pictures" (31.41-43).
The Laughing Man is the seemingly crazy guy who wanders around Miranda's neighborhood – sometimes naked – shouting things about domes and burn scales. Over time, Miranda comes to like and feel empathy for the Laughing Man, and tells her mom that he's actually just a "crazy-shaped person," instead of a truly crazy person (34.3). He is the older version of Marcus who has gone insane from traveling through time to save Sal. The Laughing Man is, at the end of the novel, a figure of sacrifice.