Underrated Haste alternate costume 
Found this fanart on pixiv. Probably was made for the fanart competition that happened a short while ago.
I can imagine him wearing this as part of an "undercover mission" in future Reingar Halloween side story. Pretend to be "just a kid" so he can find his target, but fails miserably since he still stands out because of his fashion sense and arrogant attitude, getting himself into a lot of trouble and perhaps almost failing his mission, but well, he tried.