WMG / Archipelago

Credenza is descended from Dragonfly and Olivia.

There is lots of evidence for this. Warning: Unmarked spoilers.

• There's her dad's magic (Dragonfly green), and frankly Credenza's magic (ditto). Behold the dragonfly wings.
• Olivia and Credenza's faces are very similar. And after the Art Evolution, we know that this isn't just a case of Only Six Faces.
• Credenza's dad and Dragonfly also look very similar.
• And it makes sense that Olivia and Dragonfly would be able to have descendants at all; if Dragonfly and Olivia loved each other, and she built his body, it might have occurred to her to give him ... those parts.
• Heck, even The Great Raven himself mistakes her for Dragonfly!

Tuff's inability to transform is largely psychological.

• What did Dragonfly say?
  Dragonfly: Your heart is like a watch wound too tight. If you continue like this, you will most certainly break. What do you keep so desperately inside yourself, Tuff?

Tuff is repressed and scared of hurting people in his shark form. Possibly this is why he can't transform - because he doesn't want to want it. Being a good guy, Dragonfly won't stoop to Mind Control here, but he can prod Tuff to relax.

• This would be tied to his avoidance of alcohol - he Can't Hold His Liquor, sure, but possibly because he hasn't built up a tolerance, because he's been afraid of losing control when drunk.

Steller was "overlooked" because has too much Evil Virtues to be the Raven's herald

• He's more of an evil overlord material - he doesn't want to destroy the world, he wants to rule it, maybe even not too evilly, because that's counterproductive ("promised the world in exchange for his soul"). Steller is lawful to Snow's chaotic: he cares about efficiency and things going smoothly, while Snow only wants everything to burn in an amusing manner. So, while Snow's going to do evil anyway, Steller might become a problem for the omnicidal spirit in the long run.