Well, if so far, all you've got are the normal, everyone has them ninja, like Kimimaro, I recommend saving your coupons, and when you see a ninja you really like, and would be effective for a long period of time, you should go for him/her. Like Edo Deidara, or Minato, Masked Man, etc. And Kurenai is really making a big comeback as a support character, you should invest in her, when you're not using coupons. Get her to 4 Star, then get more fragments, to refine for her skill books.