Kingston's relationship with Than is a bit hard to summarize bc it also extends to his ancestors and the previous wielders ;;o;;

Essentially Than's backstory is that it made a promise to Tsurugi to be the protector of the Crossbell family. It has been passed down through the family for generations, requiring a sacrifice to sustain its life. Its been so long that Than kind of has some memory erosion and doesn't remember details too well. Over time the Crossbell family head started seeing Than as a status symbol (Esp Kingston's Grandfather, who didn't want to use Than at all since he wanted to preserve his lifespan for as long as possible.)

Kingston was trained to inherit Than so he could be the successor, but when Kingston turned 12 he ran away from home to go on a pokemon journey against his family's approval... and brought Than on a joyride.

That journey meant a lot to Than because it finally felt like it was doing something other than collecting dust, since it got to mentor Kingston and look after him. In turn Kingston would try to teach Than about the modern world and let Than experience things that it otherwise couldn't (like letting Than possess him so it can taste various foods) and for the first time in a long while Than felt like he was in the hands of someone who saw him as a partner rather than a weapon and was reminded of his original partner, Tsurugi. Even though Than lost its memories of Tsurugi a long time ago, it never stopped longing to be with its original owner and Kingston was the closest thing it has to that.