Yūichi Jin (迅 悠一 Jin Yūichi? ) is an A-Rank (formerly S-Rank) member of Border, and part of Tamakoma Branch. He is one of the four main characters of World Trigger and the tritagonist of the series.
Jin has combed brown hair and blue eyes. He wears a pair of turquoise tinted sunglasses, usually held around his collar.
He is always wearing his Border uniform, a blue jacket with a long black stripe going down the middle of each sleeve over a white shirt and Tamakoma Branch's emblem on his jacket's upper sleeves. He also wears dark brown pants and black boots.
Jin has a laidback demeanour, even in situations when he is called out for being an antagonist, such as hiding and protecting a humanoid Neighbour. His favourite food being rice crackers, he always has a bag in his hand wherever he goes, offering some to everyone, even people he once fought with. Despite losing his mother to a Neighbor, he believes that not all Neighbors are bad guys. In fact, he thinks humans and Neighbors should become friends, showing that both races could co-exist. Jin always acts like a carefree person, but in order to protect his friends being threatened, he shows a much more serious side. Even then, he holds back when he has no desire of harming his foes, unless they force him to.
Despite his lighthearted behavior, Jin is one of the most clever and influential agents in Border. Jin is shown to be very resourceful and good at "pulling strings" with the help of his side effect, which gives him the ability to see possible futures of the people around him. He often uses any advantage or piece of information he has to obtain objectives mainly for the sake of others, especially his comrades. Jin's amiability and high rank makes him popular and well-known among Border members and counts as another advantage because of all the networking and connections he has to get things done. An example of this is when he was able to get the help of many Border members and leaders to search and destroy the thousands of trion drones that were causing irregular gates to open. He usually refers to himself as the "power elite."
Jin's relationship with his mother is unknown, but she was his only known parent before her death.
Their relationship is unknown, but it's likely Jin cared a lot for his master, as he defeated anyone in his way for Fūjin's ownership.
- Osamu Mikumo: Jin has a respect for Osamu as they share the same ideals on the treatment of neighbors, as is common in the Tamakoma Branch. Jin has saved Mikumo on a number of occasions even before the latter joined Border. He also deeply cares for this subordinate as he impacted many events during Aftokrator's Invasion which would have led to Osamu's fate as seen by his Side Effect. Thus, he deeply blames himself for Osamu and Replica's fates.
- Yūma Kuga: Jin views Yūma as his junior. He views himself in Yūma and wants him to have fun in Border, even giving up his Black Trigger, Fūjin in order for Yūma to be accepted into Border.
- Replica: Jin deeply respects Replica, calling him by the title of "sensei" (先生? lit. teacher). He also blames himself for Replica's "death" and abduction.
Kyōsuke Karasuma: Jin and Karasuma's past hasn't been revealed yet but both have been a part of Border and Tamakoma Branch for a long time. Jin admires Karasuma's strength, to the point he even left him in charge of taking care of Osamu and the other C Rank agents during the Second Large-Scale Invasion, and Karasuma views Jin as his senior. Karasuma and Jin share the same values and morals being apart of Tamakoma which suggests they have a close bond.
Jin and Tachikawa are rivals. They constantly taunt and admire each other during their battles and are constantly at odds with each other. During the assault on Yūma to obtain his trigger both combatants smiled and reminisced at the opportunity to fight again since Jin was demoted.
Despite what Miwa thinks of Jin, Jin seems to like him a lot. He even offered the Fūjin to him if he saved Osamu.
- (To Osamu) "See? Isn't it silent now, four eyes?" 
- (About the future) "Not only winning and losing decide the future." 
- (To Osamu about Aftokrator's Invasion) "As you know, I have the side effect to foresee the future. I saw various futures during the Neighbor Invasion. So when I found out they wanted to capture agents, I could've made Chika escape earlier. But that wasn't what I chose. The more focused on Chika were the enemies, the less they would prejudice the city. It worked well in that aspect, but you nearly died, and Replica is gone. So it's basically my fault, it wasn't you after all." 
- He likes Bonchi rice crackers (which he is frequently seen eating and sharing with others), women's butts and working behind the scenes.
- He came 5th in the first of the World Trigger Character Popularity Polls.
- When around Border agents, Jin is always in his trion body to reinforce his “talented elite” image.[BBF Q16]
- According to Ashihara's Q&A, if you steal and eat Jin's rice crackers, he will send a whole box of them to your room without permission in return.
- Due to brand issues, Bonchi rice crackers are changed to the generic Agesen (lit. fried rice crackers) in the anime.