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Born to a Vulcan father on Vulcan, Chiv was raised primarily Vulcan. His mother, who left before he was born, was a Romulan diplomat. Chiv was never told much about her. His father taught him the basics of Romulan culture and allowed Chiv to educate himself on it as much as he wanted, but they were pretty much fully immersed in Vulcan culture.

Despite Vulcans' claim to supress emotions, Chiv was an outcast among other children because of his Romulan heritage. He grew to be ashamed of it and repressed it, internalizing all those negative feelings. Which of course led to an unstable condition that was hidden very well.

When his father died, he left Vulcan right after and joined Starfleet. He lets everyone thinks he's Vulcan because he's still dealing with that internalized hate that makes him ashamed. He's got a lot of issues going on, but he is very experienced at hiding it all.