Obanai is your older brother,but you always treated him nicely unlike the rest of your family. When he was stuck in a cage as a child,you always brought him bread and water to eat. And you always kept him company so he wouldn't feel lonely. When he ran away from the clan he brought you with him,and you both got saved by Shinjuro (Rengoku's dad)

When Obanai became a Hashira,he took you in as his Tsuguko/student. And he always tries to spend as much time with you as possible when he was free