I didn't notice until halfway through the second one here, but both of these do involve walls. Therefore, I'll kill two stones with one bird and post them in the same thread.
If characters are literally unlocked, sealed inside a steel room or box until the requirements are met... then this illustrates how things must be going on in my Mario Kart Wii save data: Only Mii in Mario/Peach clothing and Baby Daisy remain, and they'll remain for a long, long time.
This takes a bit more explaining. There's this "dojo" in Pokémon Mystery Dungeon 2. The highest level I can access is the Ghost Dungeon, which I found inhabited by Misdreavus and nothing else. I must have never encountered a ghost type in either the first or second game until then, because they really surprised me when they passed through walls. When they're inside walls, they can't be attacked by ordinary moves. Long hallways are a nightmare. Despite the negativity this image puts forward, it's actually a great place for level grinding. On the fourth floor the Misdreavus gave out 390 experience points to everyone each time...