Obanai is deeply in love with Mitsuri, as he has stated in the manga. He feels as if he can't tell her, however, this is because of his "filthy blood", caused by the trauma he had endured in the past. He first saw her when she stood under a sakura tree, which is where he fell in love with her. To show his affection, he gets her gifts, such as food, or even the green socks that she wears with her standard uniform. Even though she is the Love Hashira, she is unaware of his infatuation, causing his love to be unrequited. They also frequently exchange letters with each other.
He is very observant of Mitsuri, as he is immediately able to tell when Mitsuri is feeling down, as well as what food and how much food she normally eats.
When fighting against Muzan with the other Hashira and Demon Slayers, Mitsuri is heavily injured. Obanai instantly rushes to her side, taking her away from the battle to get help. As he leaves her with the other Demon Slayers to go back to the battle, she is seen screaming his name, begging him to come back alive.
After the battle with Muzan, Obanai is seen cradling Mitsuri's body, both of them aware that they were going to die soon. Obanai and Mitsuri reminisce about the first time they met at Ubayashiki's mansion and how Mitsuri saved him by making him feel like a normal young man. She worries that she was useless in the end, but he says he won't let anyone say anything bad about her. Mitsuri confesses that she loves him and that the meals she ate with him were the best. She also states that he has always been looking at her so gently and asks him to take her as his wife, if they are reborn as humans. Obanai replies that if she's fine being with someone like him then he'll definitely make her happy and that he will protect her for sure. This vow eventually happens as their reincarnations are married to each other.
During their first meeting, Mitsuri thinks to herself about how Tanjiro continues to protect his sister, despite how Nezuko was turned into a Demon, calling it beautiful sibling love. In the beginning of the Swordsmith Village Arc, Mitsuri is seen running down the hill from the hot springs, complaining about a boy who was mean and who ignored her when she asked for his name. Tanjiro comforts her by telling her that supper will be ready soon, and she stops crying and rushes down towards the village.
Mitsuri likes Nezuko, as she is seen playing with her during the Swordsmith Village Arc.
Shinobu and Mitsuri get along well as they are the only two female Hashiras of the Demon Slayer Corps after Kanae's death. Mitsuri looks up to Shinobu and trusts her enough to tell her about her worries.