Celestia would probably pick up Kokichi's drinking habit of tons of panta :)))

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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-

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Celestia would probably find Kokichi to be annoying most of the time, but would probably murder anyone who hurt her brother

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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*

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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.

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(I dunno. Some random things I came up with. PROBABLY REALLY DUMB BUT STILL I THINK IT'S CUTE??)