I think the answer is usually, someone who has skills that you don't. When I think about looking for a co-founder, I know what specific skills I am looking for,

I think the answer is usually, someone who has skills that you don't. When I think about looking for a co-founder, I know what specific skills I am looking for, there is a gap, this second person would fill that gap. Some other thoughts: - Specific skill set (what is their expertise) - Values alignment - Ways of working (work life balance, etc) - Communication style - Conflict resolution and disagreement management