Pretty sure it's part of his hair on back- possibly even longer than those on sides- tied in one point and hold up with a kind of hair slide thingy- and power of anime character's law of fabulous hairstyle.

I tried that irl- on myself, since I have hairstyle- and same color- as time-skip version of Scarlet- it's possible to make this hairstyle, just gotta be more talented than me at this u.u

Oh btw- here's some pro cosplayer (photos found long time ago on forum here-dont have better quality), so you can see, it's just hair:

btw that cosplay of mains is super-great <3