There are four endings in Risky's Revenge (which to be honest aren't that different aside from the ending images). You'll get the Mighty Shantae Champ achievement for getting all of them. Here's how to get each individual ending:
1. Beat the game without collecting everything and with a total playtime over 4 hours.
2. Beat the game with everything collected and with a total playtime over 4 hours.
3. Beat the game with everything collected and with a total playtime under 4 hours.
4. Beat the game with a total playtime under 2 hours. It does not matter if you have everything or not.