As you know if you've played it, every single character in Dr. Mario 64, with the lone exception of Mario, is exclusive to Wario Land 3.
Do we have any info as to why this is? Don't get me wrong, I think it's awesome--in fact I bought the game for this reason alone--but it seems an odd choice. What's the connection?
Perhaps as the game got a relatively limited release they didn't want to bother with developing new characters and so they just picked a recent game's characters?
Personally I don't think there is any connection at all, this is probably just a coincidence as they more or less seemed to wanted to add characters.
Puzzle games like Dr. Mario later on like say during the snes/genesis era had other characters you fight along side with. You might be familiar with say Mean Bean Machine or Kirby's Avalanche in which you fight a series of characters.
