The character blends into the background in both forms, because parts of them are as dark as the background. You should make a transparent background anyway - the background here doesn't seem to contribute much to the piece, and the rules ask for transparent backgrounds in those cases.
I think overall the animations looks good and reads well. I think you might've over-animated the idle stances, there's a lot of movement that doesn't feel natural. In the t-shirt form, the character's legs grow and shrink by a pixel, which doesn't correspond to any natural human movement. If you want to have the character bouncing with their legs, have the bounce start in the feet, not the thighs. In the sworded form, the character appears to float up and down in some frames, but it's too fast to register as anything but "I think they're bouncing a lot". I think keeping them stationary might look better, since the sword-swinging and breathing animations are dynamic enough on their own.
Yeah I think making the idle stationary seems to be a better idea since this piece is super low resolution. I keep thinking that I must have a breathing animation but then it ended up weird as hell haha. Thank you for the feedback :D