[papersak]

I was missing Pluto for the longest time, too. I'm pretty sure I unlocked it playing against Luis in Disco's story. I think that was cause it was the second song to choose from and I'd overlooked it...
If that doesn't work, I'm out of ideas. :x

And I thought Relinquish was unlockable just by completing the song, regardless of whether you beat Disco or not. Got an unlock message after that, anyway...