Hmmm, I wouldn't say there's any hard rules per se, but some traits do help.

Appearance-wise I like characters who lean towards nerdy or dorky. Glasses are a big plus as is curly loose hair. Usually with a small/thin complexion. Freckles are a nice bonus, too.

Personality wise, I tend to lean towards characters who are reserved and maybe even a bit shy. Characters who are caring and kind deep down even if it doesn't always show.

Overall, the biggest factor probably is the role they play in their series and how their character arc develops. What works and what doesn't is very story dependent so it's hard to pinpoint a specific detail, but at some point during the series I'll find myself rooting for a particular character and I'll know I found them.