|“||Those who can't win even after putting in all the effort are just weak guys not worth the hassle||”|
Back in high school, Nagi seemed to be a loner who enjoyed being on his phone but later met his long term friend, Reo Mikage, and started playing football with him casually in exchange for money. His main goal is to play football and keep finding strong opponents to play against to satisfy his curiosity and test his ever growing skills.
When he first arrived to Blue Lock he was a member of Team V and is the best player in his designated stratum. He is currently ranked #6 out of the original 300 forwards chosen to be part of the Blue Lock Project and is currently starting as an offensive midfielder for the Blue Lock Eleven team against the Japan U-20.
Nagi is a noticeably tall teenager being taller than Yoichi Isagi yet younger than him. Nagi is shown to have short white hair that leaves a V-shaped fringe in between his eyes like Isagi. He has teal colored eyes with large irises.
Back in high school, Nagi wore a school uniform consisting of a white blazer with a cascading layer star pattern on the collar though he wore a dark hoodie under it instead of a dress shirt and tie. He wore the dress pants and dress shoes as well.
At Blue Lock, Nagi wears the standard-issued body suit (black with gray stripes) and tracksuit. During First Selection he wore Team V's #11 jersey, and during Second Selection wore Team White's #7 jersey.
Nagi is an unnaturally talented soccer player with amazing reflexes, good speed, and jumping skills yet despite this, he initially does not want to put in any effort into soccer such as training and belittles those who are weak and talentless at the game. He does not even know much about the best soccer players in the world, much to the chagrin of his competitors. Initially, he is shown as indifferent to playing soccer and becoming the best striker, and would rather play video games or skip training.
However, after playing in the match against Team Z, Nagi starts to take initiative and tries to score goals on his own. This only happens because he notices that his teammate, Reo Mikage, seems to be at a loss against the opposing team. Despite his initially indifferent exterior, Nagi strives to improve his soccer skills as a result of his team's loss to Yoichi Isagi. This drive to improve follows him in later arcs, and he even tarnishes a friendship because of it.
Nagi does seem to care about his friends and teammates, especially Reo Mikage even though their relationship turns rocky from the Second Selection onward.
- Creative Midfielder: Creative midfielders are midfield players that are able to create goal-scoring opportunities for the team often in situations where nothing seems likely. They usually have good ball control, passing ability and movement and will be blessed with vision and on-the-ball intelligence.
With Nagi's, perfect ball control, flexible movements, and technical ability he is able to breathe life into even the most difficult or hopeless plays. He alone has made the most outrageous goals in Blue Lock due to this skill and he continues to evolve into a creative playmaker.
Perfect Ball Control: In football, to be in control of the ball is of great importance to every level of player. The ability to control an awkward bouncing ball quickly and effectively gives the player with the ball the immediate advantage. The first touch is often the difference between success and failure in most situations during the match. Nagi has always had exceptional ball control and demonstrates it on a constant basis by setting up traps and scoring with near pin-point accuracy. He constantly scores high level shots, especially in unfavorable positions, when under pressure from tough opponents. One of his most impressive feats is when he performed a fake volley shot. His ability to control the ball is only rivaled by Itoshi Rin and Tabito Karasu and is one of the few things that escapes Rin's logic.
Trapping: Since before Blue Lock, Nagi has had exceptional trapping ability, having the ability to trap a cell phone mid air while jumping down a flight of stairs. This feat in itself was so impressive someone was willing to pay him to continue doing such things. He can trap with his entire body and with deadly efficiency, constantly finding ways to turn a bad situation into something favorable. He can trap virtually any object and he often uses his trapping to manipulate and confuse his opponents when making a run down the field; when mixed with his spatial awareness he is able to place himself at the best point possible for amazing traps and follow up shots.
- Creative Trap: When Nagi approaches the ball with a defender behind him, he can use his first touch to flick the ball over their shoulder and dodge past them. Isagi described this as a creating trap, which blows life into a play.
- Black Hole Trap: Alternatively, if Nagi is not pressured by his opponent he can use an absorbing trap, catching the ball in mid-air and killing it's momentum. This gives him the time and space to set up his next play.
- Trap Shot: This is his most common skill and he frequently uses it in his matches. Nagi first performs a trap with the ball, whether it is in motion or not. From this point he kills the velocity and momentum of the ball so he can manipulate the ball and make a pinpoint shot that breaks the defense of the opposing team and scores an effortless point. Nagi's most impressive trap shot is during the Japan National Representative match, where the ball comes falling out of the air and Nagi is marked by multiple defenders. Nagi takes control of the ball by pretending to shoot it but in actuality he is trapping it, bringing the ball back to his non dominant foot. He uses his non dominant foot to tap lift the ball giving it some air time, while he does a quick jump turn and shoots the ball between the defenders with incredible force.
- 2 Stage Fake Volley Shot: First performed in the rematch between Isagi's Team White vs. Rin's Team Red, when Nagi was pressured to score and prove his worth on the field in such a high level game. Nagi starts by setting up a high powered shot for an incoming pass but at the last second changes the angle of the shot to send the ball spinning in the air. The speed of the ball's spin makes it float in place, allowing Nagi to follow up with a high powered shot. Rin himself could not predict or counter the move and it was good enough to provoke him into getting serious. Anri when she first saw it described it as a miracle but Ego suggested it was a move possible because of Flow State.
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.
Nagi has entered Flow State numerous times in Blue Lock sometimes intentionally and others times unintentionally but he has still yet to master it. His most impressive feat in Flow State was the Fake Volley Shot.
- (To Reo, Chapter 23) If I have to work hard, I think football may be an annoying sport then. 
- (To Kuon, Chapter 23) Those who can't win even after putting in all the effort are just weak guys not worth the hassle. 
- (To Reo, Chapter 43) This is the first time I've felt like this, and in order for me to understand this "frustration".... I want to play football with this guy. I want to try.... to put in some effort 
- (To Himself, Chapter 47) I hate feeling frustrated so I don't want to lose anymore. It's not fun... unless I win. 
- (To Himself, Chapter 117) Nice to meet you Japan... I'm Nagi Seishiro.
- He plays as the offensive midfielder in the Blue Lock Eleven.
- He is able to trap multiple things, even those unrelated to soccer, like Barou's shoes.
- His favorite manga is Bonobono, a Yonkomanga series created by Mikio Igarashi 
- His favorite color is gray.
- His favorite drink is lemon tea.
- His least favorite food is crab.
- In volume 8, chapter 69, Nagi held a "Trap-Whatever-You-Can Championship".
- His given name, Nagi (
凪?), means "calm, lull".
- His surname is Seishiro (
誠 士 郎?):