Osamu Mikumo is one of the main protagonists of World Trigger and is a member of the Border with low Trion level. He joined the Border after Jin saved him. He is usually referred as simply "Four Eyes" by people outside the Mikumo Squad. He is later promoted into B-Rank and later became a Shooter and the leader of the Mikumo Squad.
Osamu is a very honest person, as noted by Yuuma Kuga who has the ability to discern lies and is thus very trusting of Osamu. Osamu is very determined and courageous always going out of his way to help others even if it puts himself in harm's way. He has a strong sense of justice. It is implied by Yuuichi Jin that his reason for joining Border is to help others(in particular Chika Amatori).