The other 'one per case' minifigure in the K'Nex Super Mario Series 4 is Waluigi. But I don't think he's as popular with collectors as Bowser Jr. He's certainly not as cute. I've actually found him a couple times, but only bought him because at the time I found Bowser Jr. there was a BOGO 40% off sale on these guys.
No respect, I tellya.
Waluigi is real easy to find via the squeeze method due to those long legs. In addition, his code begins with a double underlined 0. It's that simple.
Now that I have Waluigi, I regret not having picked up Wario from a previous K'Nex minifigure release. Oh well, I'm sure he'll be re-released in a later wave, as both Bowser and Donkey Kong have had multiple releases.