Who is Marcus from Something Blue and what is their importance?

Marcus is raised in Montana and goes on to attend Georgetown University. Marcus meets and befriends Dex in college. Marcus is not as serious about his studies as Dex and, maybe for this reason, the two only room together freshman year. Marcus and Dex remain close friends after graduation. When Dex invites Marcus to be the fifth groomsman in his wedding, Marcus is happy to oblige.