As is the nature for a demon of its kind, its current form is not what is appearence truly is, and prefers this form for an unknown reason.
As expected is of a shapeshifier, all of the whistler's clothing articles are considered a part of it's mortal phisical body, besides the necklace around it's neck. Altough it lacks reaction, truthfully, It is unknown if it can feel pain when any part of its body is harmed.
Great amounts of research have been put into finding this creature's true origin, returning with the following info, along with some theories to what presumably happened.

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The whistler in it's mortal state CAN be killed, however it is difficult AND TEMPORARY. No matter what either you or it does, it'll always come back weather or not it wishes to.
Many speculated ways to permanently decomission it have yet to yield satisfactory results.

-_______ V. Sanya