Marcus Calder is Lev's older brother and, at twenty-eight, is the oldest of the Calder children.
Marcus truly loves and cares for his brother and does not approve of Lev's tithing because of this. Marcus finally expresses this during Lev's tithing party, when he has a drunk outburst in which he calls out his parents for ending Lev's life so early and always making bad decisions, surprising everyone at the party and making Lev rethink being unwound.
When their parents decide not to take Lev back after his clapper phase, Marcus takes custody of him. Both were since disowned by their family.
When a clapper, disguised as a girl scout selling cookies, explodes in their house, Pastor Dan is killed, and Marcus is gravely injured. Lev realizes too late that the girl was in fact a clapper sent by the organization that he angered for not clapping; particularly for wasting the resources they invested to ensure that clapper missions go off without a hitch.
Afraid of what might become of him, he tells Lev to let them give him unwound parts. Their parents take care of all the hospital fees for Marcus and takes him back home with them, not given much of a choice.