What!? You've never played Castlevania? It's only the BEST action/adventure game ever. Here's a brief description:
It is an action/adventure/platforming game with a storyline about vampires and Dracula. You run around and collect items and build up the levels of your character much like an RPG. The storylines are always intricate and interesting, and the graphics and gameplay are awesome. Do a search and read some reviews. My personal favorites for the DS are "Portrait of Ruin" and "Dawn of Sorrow."