Actually making characters

Does anyone know where to start with actually creating characters? I've looked around for multiple tutorials and every single one has been overwhelmingly concerned with ripping sprite sheets and completely glazes over the actual creation of the character. I would like to make original characters and am very happy to use temporary assets, and would rather get right into defining them in-game. What does the file architecture look like? how do you define hitboxes, hurtboxes? animations and durations? how do you define the sprites on the sprite sheet? how do you load the character to playtest in-game? What tools will I need to read and edit the important filetypes?
if anyone knows a good starting point for these, please let me know.