Rantaro knows Kokichi's true self,but doesn't say anything.

They would know when one of them is sad and would try cheering each other up.

Kokichi is approximately one feet shorter than Rantaro,which is why Rantaro would tease him about it,but very friendly and not as offensive as to actually offend Kokichi like the others do.

Also,they might get mistaken for a couple,but the two are like siblings actually.

Right now,they are outside near a fountain,hanging out. (You're kokichi and male.)