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?