Many heroes are useless or have very limited usefulness. If you're just starting out, having any hero is better than no hero, but keep an eye out for heroes with a high rating in Appendix A. Focus on leveling up the highest rated heroes (being careful not to level them up too much). Once you have a handful of 4 or 5-star heroes, dismiss new lower-rated ones, and replace existing ones with new higher-rated heroes.