|“||You shouldn't aim to be the one that get's chosen, but, instead, the one that chooses||”|
Isagi is a young forward who is a second-year student and previously played as a forward for the Ichinan High School football team. When first arriving at Blue Lock he was a member of Team Z during First Selection. His main goal is to become the world's best striker.
Isagi is a fairly tall teenager with an average, muscular build. He has short dark blue hair that leaves a V-shaped fringe in between his deep blue eyes.
While attending Ichinan High School, he wore an unbuttoned blazer over a white undershirt and striped tie. Both the Ichinan High jersey and gym uniform are light-colored with black stripes.
At Blue Lock, Isagi wears the standard-issued body suit (black with gray stripes) and tracksuit. During the First Selection, he wore Team Z’s blue #11 jersey and wears Team Z's gray-capped cleats. During Second Selection, he wore Team White’s #15 jersey and that later switched to Team A's black #15 jersey during the Blue Lock Eleven tryouts. lsagi dons an official blue colored Blue Lock player uniform with the #11, that looks very similar to the ones seen in the first cover spread minus the Japan flag during the Japan National Representative match.
Isagi is easy-going and has a generally friendly, cheery personality, but his simple and innocent habits often get on the nerves of the people around him. He tends to avoid any childish squabbles among other Blue Lock competitors. At the beginning of Blue Lock, Isagi is massively unsure of himself and his skill due to being the second lowest ranked player in Blue Lock initially and his conflicting ideas of football. As time goes on Isagi develops a more assertive presence on and off the field, becoming the foundation for Team Z and garnering the respect of some of the best players in Blue Lock who mostly have heavily assertive or dominant personalities. As of Third Selection, Isagi has the confidence, charisma, and reputation to talk openly with Rin Itoshi, one of the most talented yet difficult person in the Blue Lock Project.
On the field in the beginning, Isagi was unsure on how to proceed in Blue Lock due to the conflicting ideas of football in his head and unsure on how to be egotistic but after Team Z's first win, Isagi becomes more okay with being cold and ruthless. As times goes on he is willing to step on the plans and dreams of his opponents and teammates in order to score a goal and go further in Blue Lock. When it comes to football, he is ambitious and calculated, willing to change and adapt, to get the right “chemical reactions” out of his teammates, to stay a step ahead of their opponents, and ultimately be the best player on the field, staying true to his commitment to becoming the world's best striker.
Ever since meeting and subsequently losing to Rin Itoshi, Isagi has been chasing Rin, attempting to copy and surpass him at every turn. He improved his abilities in order to compete with Rin and he goes out of his way to get closer to Rin and copy his routines in an attempt to get closer to his level and get closer to becoming the best striker in the world. But no matter how far Isagi gets in his football career he is still prone to being nervous before big matches like during the end of First Selection or at the beginning of Third Selection or the Japan National Representative match but he always seems to work through it.
- Playmaker: A player who controls the flow of the team's offensive play, and is often involved in passing moves which lead to goals, through their vision, technique, ball control, creativity, and passing ability.
Isagi creates ideals plays and chance opportunities by using his spatial awareness to view the field as a whole and the players on it. With his understanding of all the players he uses his pristine passing skills, footwork, and teamwork to create ideal plays for his teammates, chance opportunities using his teammates, and to score high level goals for himself. Due to his relationships and game time with most of the prominent Blue Lock players, Isagi is able to bring the best out of his teammates by either forcing them to perform plays so he can attack or by them pushing themselves to exceed Isagi's evolving skills. He has been noted to be the heart of Blue Lock by Sae Itoshi and implied to be that by Ego. His playmaker skills are highly recognized and respected by a majority of Blue Lock, even Jinpachi Ego comments publicly on his ability.
Expert Spatial Awareness: Isagi instinctually begins to analyze the entire field, giving him a vast understanding of the game, including his opponents and teammates’ weapons. His premonitions then allow him to envision an ideal play mid-game. His spatial awareness ability rivals and sometimes surpasses that of Rin Itoshi, the #1 player in Blue Lock.
- Goal Scent: Using his spatial awareness, Isagi instinctively recognizes the “scent of a goal” that is emitted off of key players from his prediction and is able to expertly send them the ball so they may score.
Off the Ball Movements: A term used to describe the moments where players are not in possession of the ball. Isagi may use these moments to position himself in his opponents’ blind spots, position himself in a place where he can deceive the opposition and be a decoy for his teammates shots, or to position himself close enough to a teammate to pull off super close back to back passing that can breakthrough an enemy's defense.
Direct Shot: Isagi first assesses the positions of defenders and the trajectory of the ball, then lands a powerful kick on the ball's impact point in order to score a clean goal. In doing this, Isagi avoids having to overthink his positioning and next move and can just score a clean shot with no interference. It was noted by Nagi that Isagi has no wasted motion when performing his shot, implying that Isagi performs it with flawless technique and control.
- Back Heel Shot: Isagi first assesses the positions of defenders and feints a direct shot in anticipation for his opponent's block. Then he lands a powerful backwards kick on the ball's impact point in order to score a clean shot. This move was first used against Itoshi Rin, during their rematch in the Second Selection, which surprised everyone on the field.
Flow State (Incomplete): Flow, as described by Ego, is the state of "total immersion" or "being in the zone". It is the mental state in which a person performing some activity is fully immersed in a feeling of energized focus, full involvement, and enjoyment in the process of the activity.
Isagi has unintentionally entered Flow State numerous times during Blue Lock, as early as the First Selection but he has yet to master it. His most impressive feat in Flow State is stealing a goal from Rin and Ryusei and performing a direct shot between them. The goal itself should have been impossible due to Isagi's physical stats but he still managed it albeit barely.
- (To himself) Football is a sport played with 11 people. So, one person can't win on their own. Football isn't just for one... One for all... I wanted to win! 
- (To himself) I didn't come to trample on others... To become the best in the world!! I came here... So I need to defeat those who are stronger than me! Or else, Nothing will change!! 
- (To himself) Inside of me? I wanna know more! What my monster really is. That could be my one clue to surviving inside blue lock. 
- (To Niko) But this is my win Niko... You don't have what it takes to be a striker. 
- (To Baro) No matter how much you try to understand other people's hearts. People aren't able to change others!! Watch Barou!! This is me adapting to you!! The darkness that you created on this field I'll devour it and turn it into light.
- (To Reo) You shouldn't aim to be the one that get's chosen, but, instead, the one that chooses. 
- (To Reo) Only people who will become the best strikers in the world... can survive in Blue Lock.
- (To himself) I can't become the best striker in the world that way!! I wanna win against Rin on my own!! 
- (To himself) The difference between me and Rin is the difference in our resolve to accept. I sure am lame. I thought I'd finally reached Rin. No I even thought I'd won against him. I lost to him, and was just picked to go on! I have to accept that reality!! Otherwise, there won't be a next time. I have to incorporate that piece called "Luck" in my thought process. I have to update my understanding on a fundamental level!! In order to win against Rin, I have to become a newer version of myself!! 
- (To Ego) Who the fuck cares about that....I don't give a shit. Not about the future of Japanese Football nor about some safe for for me. What I want...what we want, is to win this game.
- The name Isagi (潔) means "undefiled, pure and righteous". Notably, the kanji's adjective form, "isagiyoi" (潔い), also means "sportsmanlike, manly and brave".
- Isagi's first name Yoichi means "world" (世) (Yo) and "one" (一) (ichi).
- Isagi Yoichi can therefore be interpreted as "the world's rightful" or "the world's purest".
All trivia comes from the "1 Question, 1 Answer" tweets posted on the official Blue Lock Twitter account.
- He started playing football at the age of 4
- His favorite season is autumn.
- He likes kintsuba.
- His favorite animal is lobster.
- His hometown is Saitama.
- He is good at physical education and art. His weakest subjects are math and science.
- His foot size is 27 cm.
- His jumping height is 61 cm.
- His favorite color is light blue.
- His favorite body part is the thighs.
- He is happy when he is praised and when someone gives him sweets.
- He believes that the best way to spend a holiday would be playing soccer games and reading mangas.
- The last thing he asked from Santa for Christmas was a PlayStation 4.
- If it's his last day on Earth, he will drink some delicious tea with some kintsuba (sweet).
- His favorite actor is Vijay.