While Jujutsu Kaisen might not be the most kid-friendly anime out there, the show does contain some surprisingly wholesome relationships and interactions between its characters. One particularly heartwarming detail is the reason Itadori calls his mentor “Nanamin,” instead of his actual name, Nanami.
On paper, it would seem very unlikely that Itadori Yuuji and Nanami Kento would get along with each other. The strictly by the books, ex-salary man, jujutsu sorcerer and the wildly unpredictable, rough around the edges vessel of Sukuna are polar opposites. And yet, both characters have come to reach some mutual level of respect, and even fondness for each other.
The duo was introduced to each other by Gojo Satoru to investigate the case of some mysterious transfigured humans. While on the mission, Nanami taught Itadori some basic jujutsu principles, including how to perceive the residual cursed energy left by a cursed technique even hours after it had been activated. He also taught him the benefits of revealing one’s cursed technique to an enemy and showed him firsthand the power boost that would follow.
It’s no wonder that when the two were about to split up, Itadori tried to call him “Sensei.” Nanami rejected the title, so Itadori suggested “Nanamin” instead, prompting Nanami to threaten his charge with a slap. But how exactly did Itadori come up with the new nickname?
Since Nanami is his superior, the proper way for Itadori to address him would be “Nanami-san,” as most of the other characters do (save for Gojo, who also opts for "Nanamin.") The formal honorific probably seems too serious to Yuuji, given his playful personality, but he’d still need to accord his superior the due respect.
“Nanamin” takes care of both issues. In Japanese culture, children are allowed to make play with adults’ names, but only those they share a close relationship with. Itadori shortens “Nanami-san” to “Nanami” as a way of showing respect to the Grade 1 sorcerer, while still indicating his fondness for him.
The two have since continued to work together on occasion throughout Jujutsu Kaisen and they make a really great team. Together, they beat the Special-Grade Cursed Spirit, Mahito into submission and forced him to flee or risk getting exorcised. They’ve also saved each other’s lives a number of times. Nanami hasn’t made good on his threat of slapping Itadori yet, even though his disciple has called him "Nanamin" since coming up with the name. Perhaps, he actually doesn’t mind it at all.
