7. Samus (Super Mario RPG)
It is such a shame that Super Mario RPG never got a UK release back when it first came out. While the game is now available on the Virtual Console, PAL players of the era missed out on a superb RPG that brought the best out of Square and was the precursor for the Paper Mario games that we see today.
The game was full of little in-jokes and references, so it is perhaps no surprise that a few Nintendo characters found their way into the title. However, they weren't all just from the Mario universe and it was the appearance of a particularly famous bounty hunter that caused the biggest stir.
If the player takes a little bit of time to explore the Royal Castle, rather than simply carrying on with the game, they will come across a bedroom where none other than Metroid's Samus Aran is having a kip. Apparently she has travelled to the Mushroom Kingdom to rest up before a battle with Mother Brain.
Just why she had to go to the Mushroom Kingdom for such a fight is never explained and the character isn't seen again, robbing players of a Mario and Metroid crossover that would surely be one of the best games ever released.