Idia says that his hair is not usually hot, even offering to let the prefect touch it to test for themselves.
He says, “it can get that way when I’m mad,” so maybe when his hair turns to normal fire-color, that is when it is hot to the touch?
(He also says that his neck is cold when his hair is pulled back, but that also happens with normal, non-flame hair, so that might not be relevant.)
And the difference between the blue-flames of Twst and the fire-color flames of Twst seems to be important:
In Idia’s case the blue flames are said to be the manifestation of his family curse, and Ortho seems to suspect that the same might be true for Grim, who has similar blue flames and is under the effects on a mysterious spell.
A STYX staff member suggests that Grim’s curse and/or blessing, which may be causing his flames, “triggers when certain criteria are met,” while Idia’s father and the Director of STYX says that the ancient sorcery cast on Grim is reminiscent of the multiple layers of complex formulas that make up the Malleus-created arcane realm of Book 7.
Though Ortho says that his blue hair is also made of fire just like his brother’s, he also says that it’s just artificially generated, is not the manifestation of a curse like Idia’s, and he can change the color at will (which we see in action during Fairy Gala IF).
One of the more interesting implementations of the blue flames in the game might have been a mistake: in the original release of Malleus’ labwear vignette on JP, when Deuce asks Malleus to summon wind or fire to help dry out something, Malleus initially summoned blue flames.
This posed a few questions:
・Does Malleus have access to whatever curse-capable magic it is that torments Idia and has been cast on Grim?
・Aren’t Malleus’ signature flames supposed to be green? How many different flames can he summon, and what is the difference between them?
But it seems that Malleus summoning blue flames was an error of some kind: they have since been updated to normal flame-color on JP (they have been flame-color on EN from the start).
Blue is the color of the flames that Grim summons, and it’s possible that he’s supposed to be the only one capable of doing so, thus the correction to Malleus’ labwear.
This makes it seem like the blue, heat-less flames of the Twst-universe are unique: they’re not the flames summoned for normal flame magic (normal flame color) and they’re not Malleus’ signature flames.
The green flames that can be seen around the school pose a lot of questions of their own, but that is theory territory.
The blue flames may be a thus-far unconfirmed third thing, possibly marking a powerful curse and/or blessing that has yet to be explained.
The rules of Grim’s flames seem a little vague: he refers to it as “fire magic” and no one corrects him, despite the color, and it seems capable of burning when he wants it to despite never changing to normal-flame-color like Idia’s do.
Grim also retains the blue flames in his ears after losing the ability to summon blue fire as an attack in Book 3, so it is possible that the rules of his ear-flames are similar to those of Idia’s hair-flames, while the blue fire he can actually control has rules of its own?
If Grim’s and Idia’s respective blue flames are indeed the same, as Ortho suspects, the game might have indirectly answered the question “what happens to Idia’s hair when he showers” in Book 3: Grim goes underwater and cannot use his fire magic, but his ear-flames remain.
Twisted Wonderland Reference Note 