Bellamy - he has a scar on his left cheek that he got after the battle with the grounders. A grounder sliced open his skin with a knife while Bellamy was running from the drop shop.
Finn - there's a scar where Lincoln stabbed him with the poisoned knife after he joined a group that was on a mission to rescue Octavia.
Lincoln - he has needle marks in his neck. He got them at mount weather. They injected a drug into his neck in order to turn him into a reaper.
Jasper - there's a scar on his chest from the spear that hit him at the river. It was the second day on the ground and he barely survived the attack.
Monty - When jasper was under alies control, he stabbed Monty, leaving a scar on his stomach.
