The key thing to look at here is that he's the first ninja to do Water/Wind boosting, so you'd want to see what great combos involving those elements you can pull off.
Wind Main-has abilities that work well with kunoichi, but many preferred kunoichi tend to be water-type. He also boosts the Akatsuki girls combo (Konan and Summoner Pein).
Water Main-has abilities that empower curse seals, do poison on taijutsu, and do chaos on crit. Kidomaru is great for her, as a) his mystery can poison everyone first round, b) he can get powered up by curse seal boost and c) as a shield-type defender he pairs wonderfully with water main.
The chaos point is also worth keeping in mind, as it means he works well with 9-tails cloak Naruto and either water main or Tsunade. (I'm fond of the latter because it's always nice seeing family members get along. :D)