In the final episode of season 1 (episode 24), Shana is forced to use her (supposedly suicidal) Tenpa Josai to absorb all the Power of Existence that had been released in the battle.
Tenpa Josai normally kills the Flame Haze, but for reasons explained in the subsequent dialog with Alastor, Shana survives.
But she's naked. Why?
Is there an actual plot reason to it? Or is it just another case of forced fan-service?
Furthermore... where did that towel come from?
One of Alastor's titles is the "Flame of Heaven." In the Shakugan no Shana movie, we see his true, summoned form as a massive creature completely covered in flames, with the form of a minotaur with large webbed-like wings.
Since Tenpa Jousai uses the body of the Flame Haze as a vessel to contain the existence of the Crimson God naturally anything surrounding the container would be consumed by the intenses flames. Yuji was holding Shana's hand when she activated the summoning and came out slightly charred, but survived with the help of the Azure sustained by the his own Power of Existence.
As for the towel/blanket, it could be something extracted from Yogasa, the black cloak/cape that Shana uses to store things, which is said to be an extension of Alastor.
