Having received a letter from Tomioka Giyuu requesting that he train Kamado Tanjirou, which also explained the boy and his sister's circumstances, Sakonji set forth from his home to find the two. Upon finding them, he stops Tanjirou from attempting to kill a Demon with his knife, stating that the boy will be unable to kill the creature with such a dull weapon. Admonishing the youth for his question of how to properly defeat the beast, the elder watches as Tanjirou struggles to come up with a way to dispatch the creature. Sniffing the youth, Sakonji determines that he is no good, as he still has the scent of kindness even when confronted by vile demons.
While Tanjirou is still struggling to kill the demon, Sakonji buries the victims of the creature, afterwards kneeling by their graves with his hands together in prayer. He then introduces himself to the youth, asking if he is the one Giyuu spoke of in his letter. Getting an affirmation to his inquiry, Sakonji goes on to ask the boy what he will do if his sister decides to feast on a human, slapping him when he hesitates to answer. He admonishes the youth on his slowness to answer, stating that he lacks determination, and stresses that he must not allow his sister to harm another human under any circumstance. Getting an answer of conviction from Tanjirou, the elder agrees to test the boy to see if he is worthy of becoming a Demon Slayer, telling him to bring his sister and follow him.
Sakonji then swiftly makes for his home, with Tanjirou struggling to keep up with his pace. Arriving at their destination, the elder tells the exhausted youth that the test begins now, instructing him to hike up the mountain. He travels with the boy part way up the mountain, telling him that he must make his way back to his home before dawn. He then vanishes into the heavy fog surrounding them. Later, having made his way back down the mountain, Sakonji drapes a blanket over Nezuko, before noticing a noise at his door. A heavily exhausted and injured Tanjirou opens the door, prompting the elder to recognize his abilities, and thus his passing of the test.
Sakonji explains his profession as a Cultivator to Tanjirou, as well as how to join the Demon Killing Corps: surviving the Final Selection. To prepare him for this trial, and to see if he is worthy of even being able to participate in it, Sakonji begins to train Tanjirou in the way of the sword. He explains the intricacies of the weapon and the forces behind it, sparring with Tanjirou to improve his endurance using only his bare hands. Alongside this, he trains the boy in proper breathing techniques, slapping his stomach when he fails to get it right.
When Nezuko falls into a coma, Sakonji calls for a doctor, who cannot find anything wrong with her. Finally, after a year has passed, the elder tells Tanjirou that he has nothing more to teach him, instructing him to prove himself by improving the basics he has been taught. He then leads the youth to a huge boulder, telling him that if he is able to cut it in half, he will be given permission to go to the Final Selection. When Tanjirou hesitates, overcome by doubt, Sakonji leaves without a word, his hand in the boy's training completed.
Much later, after Tanjirou finally succeeds in slicing the boulder in half, Sakonji explains his reasons for not wanting Tanjirou to go to the Final Selection, complimenting the youth on his accomplishment. He then wraps the youth in a hug, urging him to come back alive from the Demon Killing Corps' test. Before Tanjirou leaves, the elder cuts his hair and gives him a mask to ward off bad luck. In addition, Sakonji explains that Demons increase in power by consuming humans, can use a type of sorcery to alter their bodies, and that Tanjirou will be able to figure out how many humans a Demon has consumed if he increases the sensitivity of his nose. When Tanjirou wishes him farewell, asking the old man to look after Sabito and Makomo, Sakonji wonders aloud how the boy knew the names of his two dead apprentices.
After Tanjirou returns from the Final Selection, Sakonji and Nezuko embrace him, the elder weeping tears of joy while saying that the youth had finally made it back alive. Fifteen days after this, Haganeduka arrives with Tanjirou's new sword, but ignores the youth's attempts at hospitality, prompting Sakonji to note to himself that the swordsmith hadn't changed. He then agrees with Haganeduka when he notes that Tanjirou's blade may turn red, watching shortly thereafter as it instead turns black. Noting this, the elder watches as Haganeduka throws a fit over this unexpected development.
Sakonji then watches in silence as Tanjirou buttons up his uniform, and subsequently shows him a box made out of "Mist Cloud Fir", which he can hide Nezuko from the sun with. He speculates that Tanjirou's sister may have fallen into her coma because she was attempting to regain strength through sleep, rather than through consuming humans. He goes on to tell Tanjirou that while she was sleeping, he gave Nezuko some suggestions to her subconscious to defend humans as her family from other demons. He also informs his apprentice about Caster Demons and the monster responsible for the creation of other Demons and the murder of Tanjirou's family, Kibutsuji Muzan.