|“||...It's nothing so noble. I just think... If I ran away from what I think I should do even once, then surely, I'd end up running away when it matters most. I know the kind of person I am. I'm not doing it for others. I do it for myself.||”|
Osamu Mikumo (三雲 修 Mikumo Osamu?) is one of the main characters of the manga and anime series World Trigger. He is a modest high schooler who had originally joined Border in hopes of protecting his friend Chika Amatori, but soon becomes the Captain and Shooter of Tamakoma Second.
Osamu is a boy of average height with medium length black hair and glasses. For school, Osamu wears a blue suit top and gray bottoms traditional of Japanese high school students. When not in school or his Trion body, he usually wears a sweater and a striped scarf.
When in Trion Body as a C-Rank Agent, Osamu gained an all-white suit with Border emblem patches on the shoulders with a long yellow stripe going down the middle of each sleeve. In his B-rank attire Osamu has a blue jacket with a guard on the left shoulder containing the Tamakoma Branch emblem. His left sleeve has a thick black stripe running through it, with a thinner, white stripe in the middle of the black one ending in the shoulder guard. His Squad's emblem is located on his left chest.
Osamu is a very honest person, as noted by Yūma Kuga, who has the ability to discern lies. Highly determined and courageous, with a strong sense of justice, Osamu always goes out of his way to help others, even if it puts him in harm's way. It is implied by Jin that his reason for joining Border was to help others. From his side, Osamu doesn't believe his actions are particularly noble or benevolent; rather, he doubts his own drive on principle, and believes that if he ever compromises, he'll start running away from every situation, including ones that matter. This mentality leads Osamu to genuinely believe that his actions are selfish in nature.
Osamu is aware of his limited talent for direct combat and his inadequacies compared to his teammates, or other Border members, and strives to improve upon his weaknesses. Kei Tachikawa considers him a ruthless individual, as he uses crafty and opportunistic strategies to win battles. Kōji Oki notes that Osamu deals with setbacks with a level head, rarely panicking or hesitating in the face of adversity, even in situations with his life on the line, such as his gambit during the final act of the Second Large-Scale Invasion.
Sōya Kazama's advice that Osamu fulfill his duties as a captain had an impact on the Shooter, changing his approach to make his team stronger rather than focusing on himself. This change in thinking has allowed Osamu to think outside the box and strengthen his team through ambitious ideas such as asking Jin to join his team, agreeing to allow a formerly hostile Neighbor to ally with them, and using negotiation tactics such as controlling the flow of the conversation by implanting "agents" in a room, a skill he learned from Eizō Netsuki when it was used against him in the press conference after Aftokrator's Invasion.
- (To Yūma) "Even though my chances of winning are slim, I still can't run away." 
- (To Yūma) "It's nothing so noble. I just think... If I ran away from what I think I should do even once, then surely, I'd end up running away when it matters most. I know the kind of person I am. I'm not doing it for others. I do it for myself." 
- "Forks in the road don't wait for us to choose. If I wait until all my ducks are in a row, I might never get anything done in my lifetime."
- "I'm not a hero. I can't make anyone happy with any result. I can only do what I deem right with me not hating my choice at a later point."
- "I'll get them back. Your family members that were taken by the neighbors... your friends... we'll get them back. You don't have to tell me about taking responsibility. It's the obvious thing to do."
- "Osamu," written with the kanji "修," means "discipline," which fits Osamu's personality. "Mikumo" (三雲?) literally means "three clouds."
- Osamu seems to be left-handed since he carries his sword in his left hand which is holstered on his right side. He also uses chopsticks with his left hand. During his coma-flashback in the hospital, he appears to fill out a bubble-in aptitude exam for Border using his left hand.
- He likes his parents' homemade cooking and bridges.
- According to Ashihara's Q&A, he likes his mother's handmade cream croquettes with her special white sauce the best.
- He came 1st in the First Character Popularity Poll.
- Osamu made a cameo on a special chapter of Saiki Kusuo no Ψ Nan.
- Ōji's nickname for Osamu, "Ossam," might be a pun on "Ossan" (おっさん?), which is short for "Ojīsan" (おじいさん?), meaning "old man."
- It is possible Osamu equipped Spider to the Sub instead of Main during the match against Katori and Kakizaki Squad. In fact, he is seen using it in conjunction with Raygust, which is equipped to the Main, eight times, and almost always with his non-dominant hand. However, he seems to have changed its placement again before his follow-up match.
- During that rank war, however, he again uses it at the same time as Raygust on one occasion, which should be impossible.
- Osamu is prone to feeling a heavy, anxiety-ridden sensation in his chest.