What's most important is to have complimentary skills and shared values. Lots of clues to what sharing values can look like:
1- social cues (do they say or do things the same or different?) 2- similar goals/motivations (do you often pursue similar milestones? do their motivations match or compliment yours?) 3- similar methods/reasons (do they think in lean terms? or large strategic moves? are they abstract? or ultra practical and metric driven?)
Make sure you can trust this person and that they always try to follow through on what they say. Understand how their own perception of their own status and recognition by others affects their decision making. I'd advise to watch very closely what they do, and weight what they say (justifications, rationalizations,) very lightly.