Cilan for his credit was in unenviable spot for any individual that joined the cast.
He was the character to replace Ash's best friend, mentor, and longest running travel companion in Brock. The guy who had no easy task in front did the job that was required.
You can step into the shoes, but you'll never fill them. That's why they're paired together, that's why they'll forever be linked. Just because Cilan wasn't everybody's favourite companion it doesn't mean he wasn't valuable to Ash's development as a character. Everybody made their mark in their own individual way.