Enhance “Border” around imported sprite

I am completely new to GIMP. I am using it, because I am currently creating my first 2D rpg in Unity. Currently I want to create some fighting sprites, for which I intend to adjust some of the sprites I bought on itch.io. I imported one of the sprites and changed the direction of the sword by flipping the selection I made. Now I want to draw some kind of “wind” or “draught” to make it look, as if the figure has swung the sword. Problem is, there is some kind of border around the figure and I cant draw anything outside of it ( I hope it is possible to see it on the screenshot):