There were definitely fan bases for Dixie Kong, Bandana Dee, Impa, and Chibi Robo prior to any of them appearing in leaks (I don't actually recall Chibi Robo being in a leak...).
Chorus Men, though, owe their popularity entirely due to the Gematsu leak. That trio of characters was never even considered prior to being "leaked," and once they were and it appeared that they were inevitable, fans of Rhythm Heaven united around that concept. The thing is, I think the fan base for them took them for granted up until the end, so once it turned out that they weren't playable, it resulted in the Chorus Men becoming a semi-popular candidate for DLC. I mean, after being introduced to a cool concept for a character, spending months thinking the character was in, speculating how they would play, and just waiting for their "inevitable" reveal, one would probably want them in quite a bit.
But yeah, the Chorus Men are the best examples of characters people wouldn't care about if not for leaks.