For me, the most important trait I look for in a co-founder is trust. If that element is absent, I think building a long-term partnership would be too difficult, no matter how wonderful they may be in other areas. They also need to be reliable, as you need someone who's going to stick it out through the highs and lows. Apart from that, I look for an overall high level of intelligence and someone who compliments my skill set (e.g. someone with a killer sense of design).