Colin was a resident of Seattle who was close with Aaron. After the outbreak of the Cordyceps brain infection, Colin received a letter from Aaron asking him to join him in a planned raid on the abandoned Westlake Bank. Aaron believed it still contained pre-outbreak supplies that the pair could use to trade with a group who could grant them safe passage outside of the now quarantined Seattle area.
Events of The Last of Us Part II
In April, 2038, Ellie can find Aaron's note for Colin while searching for Abby Anderson in Seattle. She can also search the bank with Dina, finding the supplies still within it, revealing Aaron and Colin never went ahead with his plan to raid the bank.