Quoting Le.:“ Think of the word 'Celtic' and you see what I mean. I like the C.”

i think C-E is the exception.

Quoting Mara:“ honestly, i thought cillian-- based on the appearance would've been pronounced "Sillian", which i don't like. killian, now, that's alright.”

This 100%  

I wanted to name BoobMan Killian, but Jesse hated it. The actor who played Scarecrow in the Batman movie before The Dark Knight, that's Killian Murphy. He was also in Red Eye and Breakfast on Pluto.

I really like the name Killian, I know a guy named Killian and I've always thought it was a cool name but to put a 'C' instead of a 'K' would just be ridiculous in my opinion... I don't think anyone would pronounce his name right or spell it right.