Middos and yiras shamayim priority, next comes compatibility, stable in mind / settled, and the rest differs for each person (looks money background health etc)
If you’re asking more specifically about character, I’d say make sure no temper, giving type – baal chesed. And oh, very important: a happy positive person – makes life so much easier…
One simple rule I’ve heard from many wise people which has been proven. Make sure they future in-laws have lots of money, and they also poses the quality of sharing it.