Calvin can best be described as an oddball whose behavior can't be predicted by any method of logic or reason. He tends to make peculiar comments during any situation and has the most unexpected fixations, but hanging with him seems to brighten one's morale. He tries to hide it, but he truly cares for the well-being of others.

Things to keep in mind: 

-Says unhinged or completely random things, often times associated with lizards, dinosaurs, or Kaiju

-Sentence structure is very stunted and abrupt at times 

-Says things that make sense in a roundabout way or make sense only to him or a select group of people, but leaves others confused

Here are some voice headcanons to help: (note: these voices are just to help guide you, we are not looking for impressions, so feel free to not follow these so closely and put your own spin on the character)

Gabe Khouth (Nicol Amalfi): https://www.behindthevoiceactors.com/tv-shows/Mobile-Suit-Gundam-SEED/Nicol-Amalfi/

Jonathan Leon (Kazushi Yamagishi): https://www.youtube.com/watch?v=8FiC6DLIzqQ&t=543s

Benjamin Diskin (Hayato): https://www.behindthevoiceactors.com/video-games/Fire-Emblem-Fates/Hayato/