Marcus is one of the few survivors that has two out of three Personal skills that benefit exploring: Powerhouse and Leadership. With Leadership skill, Marcus is a natural choice for any missions meant to improve trust, or for recruiting survivors.
Ed Jones - Marcus and Ed are friends and co-workers, being on a camping trip to get away from their jobs. They seem to show a close bond, as Ed rarely questions the judgement of Marcus, showing that Ed gives a high respect for him. It should also be noted that if Ed dies, Marcus will immediately go into Depression, in which suicide is possible, when the notification appears that Ed has died. This will only occur, however, if Ed dies following the attack by the re-animated Thomas Ritter.
Maya Torres - While scavaging and searching for other campsites at the top of Mt. Tanner, Marcus and Ed hear distant gunshots, which lead to the attacked Maya Torres. It is shown that even though the three just met, a bond between them is immediatly formed even before they arrive at the Church. Another factor showing this is that Maya opens up about herself if playing as Marcus when heading to the Church of the Ascension.
Lily Ritter - Upon leaving Mt. Tanner, the group (Marcus, Maya, and Ed) are contacted by Lily via radio. Upon describing their situation, Lily suggest they come join them at the Church, in which Marcus shows delight in. After arrival and introductions, the journal entry about Lily suggests she "seems friendlier than most around here".