Rinse and repeat until your character's movement fits what you are aiming for. Try changing some numbers around, then open up Rivals' Training Mode and test out the character.
For everything else, use the Rivals General Stats Sheet for a good point of reference. Even slight changes to these numbers can have a big effect on a character's weight and survivability. Take special care when editing your character's 'hitstun_grav' (default 0.5) and 'air_friction' (default 0.04). If you are using Sandbert as a template, it is very important that you edit these entirely, as Sandbert's stats are much stronger than the average character. See the Workshop Player Variables list to see what all of them do. 'scripts/init.gml' contains your character's base stats, and other base variables. You can also set your character's name and other information by editing the values in this file. For new characters you should erase the 'url' tag.
In your character folder, the file 'config.ini' contains important details like your character's name, description, author and item type.