To look for complementarity, seek interactions as if they are discoveries of new land. A good goal when there's a group of people (such as the girls at your school) is the "exploration goal": to learn about everyone and discover the highest level of complementarity possible.
If you have the long term goal of finding and dating a person that would be good to marry if that person exists in the group, then focusing on that is much worse in every way than focusing on the exploration goal, especially in complexity.
Edit: should have said "keep on discovering a higher level of complementarity" instead of "learn about everyone and discover the highest level of complementarity possible"