Am I the only one who finds the whistle in Mystia's theme a bit... Weird?
It's obvious - the most logical explanation can be linked to her ability and what yuuka and Eiki have to say about her in PoFL.
"The song isn't coming from inside you, but rather you're singing a song you're hearing from a faint voice." - Yuuka
"If you continue singing without thinking, the souls around you will become more than strange." - Eiki
"That may be connected to an unprecedented grave sin." - Eiki
In short.... Mystia Lorelei is a herald of Azathoth - The Nuclear Chaos, The Daemon Sultan, The Blind Idiot God.
https://en.wikipedia.org/wiki/AzathothA god of madness from beyond time and space, who sits in the center of all chaos, pulsing to an unceasing, insane song of pipes and drums.
I've always thought that Mystia has a bit more to her than is revealed, though whether or not I'd actually turn to H.P. Lovecraft to satisfy that feeling... it definitely makes for an interesting concept, however.