In Persona 4, he is a silent protagonist, as in the series tradition. Except for instances where he audibly speaks when summoning a Persona and so on, with his personality ultimately based on the player's actions and decisions. This is emphasized in Persona 4 Golden, as the Investigation Team members comment on how he doesn't talk much when equipped with the Butler Suit. His dialogue choices make him seem very neutral and include some limited sense of humor along with occasional flirtatious or silly behavior. Overall, he is portrayed through his dialogue options and actions as having a more extrovert personality than the Persona 3 protagonist, such as when both hear Jika Net Tanaka, and the former describes it as a "cheery song" unlike the latter, who describes it as an "annoying song." But while he seems to have a strong moral compass, there are several dialogue options that show he could be mean if he wanted it. In one case, when asked to stay and help clean the basketball court, the options being either to stay or to "be an ass and leave," though indicating an inclination to the former. Additionally, while the game portrays him as a calm and decisive individual, there are scenes that show him to having limits as to how much tolerance he has. One example, in particular, is the decision to kill Namatame, as the dialogue choice "Calm the hell down!" can be shown, and when selected, it demonstrates how he hardly ever shows to be this aggressive, as the Investigation Team gets surprised by his outburst at Yosuke.

He is also a decent cook, as he is capable of making omelets that taste excellent compared to the Mystery Food X that the girls made during the celebration for Mitsuo's arrest. During that event, his omelet was the only one that truly won Nanako's favor, despite Nanako claiming that the girls' omelets were "delicious" just to be polite.

In the manga adaptation, Souji Seta is a distant loner who demonstrates a high level of maturity, his overly formal speaking tone being one of the most telling signs. Seta has a pessimistic outlook on life due to his parents and having to move frequently as a result of said parents' demanding careers, but he eventually warms up as he explores friendship.

In the anime adaptation, Yu Narukami is depicted as quiet, polite and calm for most situations. Before the awakening of his Persona, Yu seemed to be emotionless and cold at times, as well as occasionally deadpan. Afterwards, however, he is shown to value the bonds of friendship between the members of the Investigation Team and is highly concerned about his friends, going to great lengths in order to help others. As he grows closer to the group he opens up more and more, and by the end, he is as unique (though still quieter) as any of the other characters.

He has a dry sense of humor and sometimes does questionable things which he thinks are funny. He also seems a bit naive and oblivious to certain situations, which somehow keeps working to his benefit; especially when dealing with the girls, as he is able to get away with things that would warrant a smack for any of the other male characters, mainly due to how little he would mean offense. He's also shown to be somewhat perverted, saying he wished that he taped "Yukiko's Hunt for Prince Charming" and repeatedly pressing the record button on his TV remote during "Rise's Daring Challenge." During the course of the story, it is revealed his biggest insecurity is abandonment, fearing to lose contact with all of his friends when he moves away from them. By the end of the show, he accepts his Shadow and acknowledges that he has to accept being separated from them when the time comes.

In the second anime adaption, Yu appears to be friendly, reserved, determined, straightforward and has a habit of shaking people's hands when he meets new people, unlike his first anime counterpart.

In the Persona 4 Arena duology, Yu is also shown to be a fairly sarcastic and witty person, occasionally making jokes at the expense of the other members of the Investigation Team; however, for the most part, he is shown to be the most mature member of the group (an example of which is the fact that attempts to target his personal insecurities by enemies, more often than not, fall flat). He has a sharp mind and is able to quickly realize the trick behind the tournament that forces the group to fight one another. He deeply cares for his friends and one of the few ways to truly anger him is to threaten either them or Nanako which causes him to become very cold, the latter not being entirely an exaggeration from his title in the tournament, when the taunts about her being captive being the one ruse he actually falls for. He also has a great understanding of the bonds of people, which leads to Aigis and Mitsuru to realize that despite their different appearances, Yu reminds them of the protagonist of Persona 3. While not above making fun of him, he trusts Yosuke completely and generally understands him best out of the entire Investigation team and vice versa, continuing the theme that the anime established that Yu and Yosuke generally lead the team side by side, as well as discussing plans with each other on serious issues.

His characterization in Persona Q is essentially a combination of the above adaptations. He's confident and affable, but also strange and a little random at times. His dialogue choices during the P4 route support this, as they're noticeably more cheery and bizarre than his Persona 3 counterpart. During the group date cafe, when Mitsuru and Yukari deem him to be boyfriend material, he visibly blushes.

Yu's most consistent trait in all of his depictions is his need to be connected to people. He views himself as a weak person and gets his strength from his bonds and being accepted by others. This is shown in the anime and by his own admission in Persona Q, as when he is separated from them, Yu withdraws into himself and loses the confidence he normally portrays.