I think it's Michonne. Those point of view shots they were showing from one of the four locked inside the van tie into the last shots, so that pretty much narrows it down to her, Daryl, Glenn or Rosita. Rosita would have no impact, Daryl would actually have very little impact on the group as he has no close relationships, Glenn would be too obvious, so I think it feasibly only leaves Michonne. I would love to be wrong though.