i've been thinking about modding the game, but i don't know the correct mod tool to use. Any help?

if you're just wanting to have basic stuff for a character (i.e. art / dialogue only), you can use the modded character examples the game comes with as a base to work off of

if you want to create custom code for the character as well though, you can browse through this thread for details https://itch.io/t/3647086/how-to-edit-a-modded-unit