|“||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 but overtime he develops a more defined and stronger looking physique. He has short dark blue hair in the manga but black hair in the anime. His hair 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 blue 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 U-20 match. As a regular for Team Germany, Isagi dons the red and black with gold stripes Bastard München uniform as #11.
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 people in the Blue Lock Project.
Initially, Isagi was unsure on how to proceed in Blue Lock due to the conflicting ideology being promoted in the program, as well as being unsure on how to be egotistic. However, 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 losing to Rin, Isagi has been chasing him, attempting to surpass him at every turn. He goes out of his way to get closer to Rin, attempting to copy his cooldown routines and adapting his mindset, in an attempt to get closer to his level. Isagi believes that when he is able to truly surpass Rin, he will be closer than ever to his goal of becoming the best striker in the world. Isagi is still prone to being nervous before big matches such as: During the end of First Selection, the beginning of Third Selection or the Japan U-20 match and was slightly nervous whilst being interviewed after his decisive goal. However, in each case Isagi was able to fully realise his ego, and confidently stake his place among the best in Blue Lock, declaring that he would lead the Japan U-20 to victory in the World Cup.
- Raumdeuter: The word essentially translates to “space investigator”, and is best used to define the German’s effective role when exploiting the half space. A Raumdeuter’s main function is to drift infield and find pockets of space in which to thrive; it is essentially a player who locates dangerous areas of the pitch and uses unoccupied space to create confusion among opposition backlines. They are limited dribblers, and are not characterized by their pace. Their first touch is very efficient, and is a huge contributor to their set-up touches prior to scoring.
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 off the ball movements to occupy empty spaces or create dangerous 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 skill. He has been noted to be the heart of Blue Lock by Sae Itoshi and implied to be that by Ego. His skills are highly recognized and respected by a majority of Blue Lock, even Jinpachi Ego comments publicly on his ability.
Spatial Awareness: Isagi instinctually begins to analyze the entire field, giving him a vast understanding of the game, including his opponents and teammates’ weapons. Based on this understanding, Isagi can make accurate predictions as to the flow of the game. These premonitions then allow him to envision an ideal play mid-game and execute it. His spatial awareness is among the highest in Blue Lock, often rivaling and sometimes surpassing other player's own visions, such as Niko and Rin.
- 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.
- Adaptability: It was noted by Naruhaya that Isagi is a genius in terms of adaptability. By understanding the core of both his teammate's and opponent's football, Isagi can adapt his style to either devour or counter their talent. The best example of this is when he "devoured" Naruhaya's off the ball movements and ability to exploit blind spots. This was also seen when he adapted to devour Baro and exploit his weaknesses in order to score. Perhaps Isagi's greatest feat of adaptation was when he was able to adjust his plan to defeat Rin, taking into account his physical advantages, and predicting the actions Rin would take to block his shot.
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.
- Positioning: Mirroring Rin, Isagi is able to use his positioning to exploit luck during a game. If he realises the ball will not be passed to him, Isagi will move to an opportune location, where he is in the blind spot of his opponent and where he would be able to score directly off of a direct shot. He used this to score the decisive goal in the match against the U-20.
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.
Reflex: Isagi, as of the match against the Japan U-20, has begun to incorporate reflex into his playmaking. Isagi is able to use his awareness of the conditions of the field to find spaces of opportunity, either to attack the opponent's goal or defend his own. He used this to instinctively move to a position where he could use a Direct Shot, after passing to Nagi, he used this to read Sae's corner kick, by taking into account Sae's pass, as well as Rin's and Shidou's positioning, and he also demonstrated this ability at the end of Japan U-20 match when he by reflex moved to the ball while in the blindspots of the Japan U-20 defense, scoring a powerful direct shot.
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, even as early as the First Selection. His most impressive feat in Flow was scoring a goal ahead of Rin and Shidou, performing a direct shot between them. In the lead up to this, he used his off the ball movements to feint and cross Karasu, as well as move into Otoya's blind spot. The goal itself should have been impossible due to Isagi's physical stats but he managed it due to combining and using all his weapons optimally at the same time.
- (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.
- His ideal type of a girl is someone who has a great smile and laughs a lot.
- He feels sad when someone ignores him.
- His hobby is strolling.