Not a ship btw.

Besties. They are besties.

Rantaro would be the first male that Tenko would be friends with.

They talk about a lot of things and Rantaro introduced Tenko to his sisters (once he finds them)

His sisters look up to Tenko and see her as a big sister.

They end up being each other's wingman so they can set each other up with their crush (depending on who I ship them with)

Like I said, they are besties