Chainsaw Man wouldn't be the first anime to introduce the student-mentor dynamic between its characters. It does, however, contain one of the more interesting applications of the trope. As a protagonist, Denji is known to break many of the conventions inherent in other shonen series, so it's only right that his mentor, Aki, would need to diverge from the status quo as well. Even though Aki and Denji are far from the norm insofar as anime mentorships go, Aki is just as great a teacher as any in shonen.
He's far from a perfect person, but those flaws only make him a better fit for Denji, who would certainly have a tough time learning to be a Shinobi from Kakashi, or studying martial arts under Master Roshi. If anything, Aki is the wisest teacher of all, because he learns just as much from Denji as Denji does from him. A great mentor isn't just one who imparts their knowledge to a student, but one who grows alongside them; and few mentors in anime history have exemplified that dynamic as well as Aki.
Aki Got Denji to Rethink His Goals
Denji's Dreams Were Juvenile and Selfish, but Aki Helped Him See The Bigger Picture
Denji's early life was a tragic one. He grew up extremely poor, with no parental guidance, and no friends or loved ones to care for him. He was forced to fend for himself from a young age, and the only one who was ever there for him was Pochita, a Devil. In fact, Denji always had a more negative relationship with people than with Devils, which is probably why he's far more accepting of them than the average person.
Denji's only interaction with other people for much of his upbringing was with the Yakuza, who used him to pay off his father's debt with them. Denji never attended school, never had any money for himself, and hardly had the bare minimum to survive. This kept him completely outcast from society, without even a basic education, let alone an understanding of the etiquette of interpersonal relationships. That made Denji someone who was easily taken advantage of, and had no goals or values of his own. Denji's only driving forces were to have food to eat and a place to sleep, and nothing he had to do to achieve those goals was off limits.
This all began to change almost immediately after meeting Aki. While Aki never taught Denji any formal lessons with the intention of guiding him, Aki's mere presence in Denji's life made Denji begin questioning things that he always took for granted. Aki and Denji's fight in the alleyway was the first time Denji had to outwardly rationalize his motivations to another person, in order to prove himself as worthy to Aki. Later, Aki once again challenges Denji when the latter kills a Fiend with an axe in order to spare it pain.
Aki reminded Denji that there were people fighting for their families to kill Devils, and that it was bigger than just his selfish desire for luxury. While Denji responded negatively to this notion at first, acting as though Aki didn't know what he was talking about, it was a notion that clearly had an effect on him. This was best shown during his fight with the Leech Devil. When Denji expressed his desire to save Power just to touch her breasts, even the Leech Devil told Denji that was a stupid dream.
Her response to his motivation instantly triggered anger in Denji due to his past interactions with Aki. This led Denji to declare he and the Leech Devil should have a "dream battle": whoever kills the other proves they have more resolve, thus proving that their dream was the strongest. Unfortunately, Denji was completely outmatched by the Leech Devil, proving that her love for the Bat Devil was stronger than Denji's desire to "touch boobs".
That wouldn't be the final word on the matter, though. Instead, it would be Aki who would swoop in to kill Leech and rescue Denji in the end, inadvertently making him the true winner of the "dream battle". Denji's dreams at this point were undeniably juvenile, but the fact he even began thinking seriously about what his dreams might be was a big deal for him. Denji never had any distinct moment of epiphany, but he did slowly show signs that he was thinking more and more deeply about his personal goals and dreams, and his early interactions with Aki were the most obvious spark for that change.
Aki Was Strongly Acclimated to the World Denji was Outcast From
Denji Always Knew How to Take Care of Himself, But Aki Taught Him The Importance of Self-Care
Other than getting Denji to think critically about his own motivations, another important way Aki contributed to Denji’s personal growth was by teaching him the basics of fitting in to society. Being that he was alone without meaningful human interaction for much of his early life, Denji had to essentially relearn to be a regular person. Aki gave Denji a family, a place to come home to, and home-cooked meals. These things were the most important lessons any mentor could teach Denji, because it allowed him to learn to trust and rely on people.
Interestingly, just as Aki taught Denji to believe in humanity, Denji taught Aki to believe in Devils. Like Denji, Aki also lost his own family when he was young. Instead of their deaths being caused by a human's hand, though, they were caused by the power of a Devil: specifically, the Gun Devil. Aki was then taken in by Public Safety, and lived the majority of his life as a member of human society as a Devil Hunter. Even though Aki owed his life to Makima, he also developed other relationships with people, like his comrades in Public Safety, and Himeno in particular.
On the other hand, the loss of his family led Aki to grow an intense hatred of the Gun Devil, making it his life’s goal to kill the Gun Devil, and all Devils like it, no matter what. While Aki outwardly despised Devils before meeting Denji and Power, by the end of his life, he had no qualms about admitting that he cared deeply for both of them. In a way, Aki found that he identified with the Devils, because even though he grew up in closer proximity to humanity than Denji did, he was still always a loner who didn’t quite fit in.
Denji had Aki's back more than once, but no single event was more important for their bonding than when Denji captured Katana Man. Aki and Denji's iconic "nut-shot contest" is infamous for its ridiculousness, but it's also an emotionally powerful moment. It shows how Denji is able to pull Aki out of his cage, thereby showing that Aki has even gained a degree of respect for Denji in that moment. Aki wouldn't step out of his comfort zone like that for just anybody.
After all, the only reason he even started smoking cigarettes was due to Himeno's influence. Similarly, Aki only ever pierced his ears because Himeno encouraged it. Convincing him to get a piercing is hard enough, so for Aki to change his entire perspective on something as central to his life as killing Devils is no small task. Of course, changing Aki's entire view on Devils wasn't something Denji did entirely on his own — Angel, Galgalgi and Power all had big roles to play in that as well — but Denji was the person who most directly challenged Aki's perception of what a Devil was supposed to belike, and how they should be treated.
It wasn't just that Aki became more acclimated to Devils thanks to Denji, though; the inverse can also be said for Denji's relationship to human society. The biggest factor in Denji learning any kind of manners or social etiquette was due to his time spent with Aki. The most obvious indication of Denji's complete turn-around was immediately after the death of Makima. After Denji killed Makima, the way he had to ensure that she wouldn't revive was by eating her, and Denji chose to do this by cooking her into various meals to make her more palatable.
Notably, the one who always used to cook for Denji was Aki, because Denji and Power were both notoriously terrible cooks when they first moved in with Aki. The fact Denji was able to cook numerous dishes with "Makima meat" shows that he truly underwent a dramatic change from his earlier life. In Chapter 165 of Chainsaw Man, Denji even reveals that he used to eat toilet paper to survive, and fans are well aware of his propensity to "swallow anything with nutritional value". Denji has come a long way from eating toilet paper and vomit, and Aki is definitely the one Denji has to thank for that.
Aki Understood Denji Because of His Past
Sometimes, Experience is the Best Teacher
While Aki and Denji's early lives went in dramatically different directions, aspects of their childhoods were similar because they both lost their families young. Part of the reason Aki is the best person to guide Denji is because he understands him after having to grow up without his family, too. In fact, though neither of them realize it, both Denji and Aki feel extreme guilt towards the loss of their families, which is a driving factor in who they've both become as people.
Aki blames himself for the death of his brother and ultimately feels survivor's guilt for being the only member of his family to have survived the Gun Devil's attack. Similarly, Denji killed his own father to protect himself from one of his dad's drunken tirades, leaving him alone and without a parent to guide him. As someone who has lived as a member of the society Denji was outcast from, Aki serves as the parental figure Denji never had, keeping him in line and even reprimanding him when necessary.
Aki is a person with a strong sense of justice and an unwavering belief in his personal values, and that's the complete opposite of what Denji is. While Aki is perhaps too unwavering, that contrasts perfectly with Denji, who is willing to waver in any direction without a second thought. This naturally leads to conflict between the two, and their constant fights ultimately force them to end up meeting somewhere in the middle, which is the perfect place to be for both of them.
Aki relentlessly puts others over himself, and it's to the point that most of his decisions prove detrimental to his own well-being. Inversely, Denji does pretty much everything for his own well-being, and even the things he does for other people are only because they align with his selfish goal. By constantly challenging each other whenever they would lean too far in one direction, both Aki and Denji pulled each other back to reality.
Denji was Also a Mentor for Aki, in a Way
Aki Learned Just as Much From Denji as Denji Did From Him
Even though Aki was able to understand Denji because of the similarity of the traumas they faced, Denji and Aki were also in many ways each other's opposites. Their opposing dispositions led to conflict at the start of their relationship, but also ended up benefiting them both in the end. Both Denji and Aki were able to show each other another aspect of life they were always missing, but it took some time before they were each able to open up to that possibility.
Aki and Denji's conflicted relationship was best encapsulated in their first and last moments together. The first time Denji and Aki met, they got in a fight, leaving Aki more injured than Denji due to Denji's nature as a Devil Hybrid (not to mention his particularly ruthless fighting technique). When reporting to Makima, Aki was forced to lean on Denji despite being beaten up by him — that's exactly the kind of relationship Aki and Denji would always have.
Despite Denji being more trouble than he's worth, Aki found himself leaning on Denji out of necessity, even though he never intended to. For Denji's part, he never planned or purposely meant to be Aki's friend; it just happened organically for him. Just as he naturally helped Aki stand up after beating him up in their first meeting, Denji served as a distraction from Aki's loneliness, giving him the thing he always needed but never wanted: somebody by his side.
Even when Aki beat Denji up to push him away, the first time they met, Denji fought back, not for Aki's own sake, but because it was what he wanted. That's ultimately what Aki needed more than anything. If Denji had purposely intended to be there for Aki, it's likely he would have refused and pushed him away more. It was only because Denji was forcefully thrust upon him that he was able to eventually accept him.
This dynamic was explained perfectly by Aki in him and Denji's last shared moment together before Aki became the Gun Fiend. Denji and Power accompanied Aki on a trip to see his family's grave; a trip that was always an emotional one for him. This time, though, Denji and Power were such a hassle for Aki that he never had a chance to focus on his sadness.
As Aki put it, "Every year, when I went to visit their grave, I'd remember nothing but bad things. It was depressing. But this time, you guys were such pests that I didn't have the time to get lost in that." Aki was always someone who spent way too much time inside his own head. Because Denji was someone who was so willing to put himself out there, just spending time around him helped pull Aki out of his shell.
Aki Was the Best Mentor For Denji Because He Wasn't a Mentor at All
Denji Needed a Mentor, Aki Gave Him a Friend
Aki was the perfect mentor and guide for Denji, but it wasn't just a one-way-street. Denji also taught Aki valuable lessons that inspired a noticeable change in him by the end of CSM Part 1. Aside from freeing Aki of his loneliness and distracting him from his depression, Denji changed Aki by teaching him to respect Devils just as he would other people.
It may very well be the case that the reason Aki was such a great mentor for Denji was that he wasn't actually a mentor at all, but rather a friend. What Denji needed more than anything was another human being to care for him and have his back, and Aki doing that was enough to teach Denji everything he needed to know about being a good person.
