Here are some BIG SPOILERS, so don't read any farther if you don't want to know:
The endings in Superstorm Melon Date basically revolve around how much you flirt vs. how friendly you are. Each of the endings is on a spectrum based on how flirty/friend you are. (You can check this in the stats screen).
Flirtiness > 81%: You and Mel have a super steamy romance, but it doesn't really last. You stay friends and hook up every so often, but you aren't a couple.
61-80%: You are Mel have a strong physical relationship. Sometimes you fight a lot, but your strong physical connection keeps you together, and you make a long-term relationship work.
41-60%: You and Mel have a really balanced relationship. Every jokes about how you're the perfect couple. It's kind of embarrassing, really.
21-40%: You and Mel can't make a long-term relationship work, but you do go into business together selling clothes that Mel designs.
< 21%: You and Mel aren't really a couple at all, but you are best friends for life.