Celestia would probably pick up Kokichi's drinking habit of tons of panta :)))
----------
Kokichi and Celestia would lie, prank, and pull each other's finger a lot. However, when they aren't against each other, they are working together to cause the end of all humanity-
----------
Celestia would probably find Kokichi to be annoying most of the time, but would probably murder anyone who hurt her brother
----------
Kokichi: SIS MEET MY BOYFRIEND, SHUICHI. HE'S WAY BETTER THAN HIFUMI BY FAR
Celestia: I DON'T EVEN LIKE THAT PEASANT AS A FRIEND YOU GAY WORTHLESS ABORTION
Kokichi: SHUT THE FUCK UP AND INTRODUCE YOURSELF YOU BITCH
Shuichi: you guys seem to have... quite the relationship...
Celestia: Hi, I'm Celeste. You hurt my brother and you'll soon find yourself slowly running out of oxygen as you're buried six feet under the ground. *smiles innocently*
----------
Mom: Who broke the pot?
Celeste/Kokichi: S/He did it!
Kokichi: well, I mean it could of been me
Celeste: who could really tell? We don't have evidence, so it might as well could have been me.
Mom: You both are cleaning it up and paying me back.
----------
(I dunno. Some random things I came up with. PROBABLY REALLY DUMB BUT STILL I THINK IT'S CUTE??)