The Flame Princess.

Flame Princess is the only character that we can predict in which episodes she will appear.Until now all of the episodes she appears (like the wikia said)is fire connected although it can be from any language.For example:

1."Incendium".In Latin it means "Fire" or "Burning"(Season3)

2."Hot To The Touch"(Season 4)

3."Burning Low"(Season 4)

4."Ignition Point"(maybe)(Season 4)