I paint the background very carefully around the character with watercolors. I used to use masking fluid but found it quite laborious so now I just try not to cross the lines. When adding splatters I cover the character with piece of paper.
No worries, masking fluid does not destroy the character. Unless you leave the mask sitting on a paper for days... then it can get stuck and might rip the surface of the paper. That has happened to me...
It can be a bit tricky to work with masking fluid at first but I am sure you'll learn quickly: apply the fluid with a brush (use an old one - the stuff destroys brushes), let it dry for a while and then paint the rest of the picture. Paint does not catch the masked parts. After the paint has dried you can remove the masking fluid. You can either scratch it carefully with your fingertips or nails or then use eraser to help you. Removing the mask is funnier than applying it.
I have used Winsor & Newton's Art Masking Fluid, here is a link to manufacturer's page: www.winsornewton.com/products.… I haven't used other brands so cannot tell if they're all similar... but I believe there are not big differences.