It was only released in Japan, but Magical Pop'n was a fantastic game for the Super Nintendo. I never understood why it didn't get a North American release. That game had so much charm and encompassed everything you could want out of a side-scroller. It has a female protagonist, simply named Princess, who has to reclaim a stolen gem (like every game of that era). It's sort of a weird mix between Mega Man, Zelda, a little bit of Castlevania, and maybe even Metroid.
The level design is fun and well thought out. The art is cute and charming. It's full of puzzles, discovery, and epic boss battles. Definitely a game I consider a hidden gem. I'm currently teaching my 4 year old niece to get through the different levels and her face lights up every time she plays it. She really loves that the Princess is the one conquering the bad guys. We all had games that did that for us at one point in our lives.
To me, Princess is THE Nintendo Princess...even though she's not technically Nintendo.
She needs an epic comeback. Smash Bros DLC, anyone? She already has a moveset mapped out. (I know it's never going to happen).