I love your style! It's so adorable ;w;
As for transparent background on Sai, I have a fix for that! Simply create a new layer and put it below all your other layers. It will act as a background. Color it in entirely, whatever color, it doesn't matter. Then turn that layer off, so it isn't visible. When you export your image as .png or .jpg, it will prompt you to choose a pixel format, one option will say "Each pixel have opacity" in parentheses. Just check that bubble and when you save it, your background will be transparent!<3