Three things make for a compelling secondary character.
Firstly, we have to have a reasonably detailed account of their appearance and habits. But beware. It’s no good just piling up quirks and random characteristics. Everything that we find out has to make them a better foil for your novel’s main character.
Secondly, we need some insight into their back story. Exercise your ingenuity to come up with indirect methods of exposition though.
Thirdly, the character needs contradictions and tensions in their life. Use this sparingly, as a bunch of highly conflicted characters can give the reader overwhelm.