if you just want mario but slower and with a double jump, you actually don't need a completely custom character.
the player's speed cap can be controlled by "Defines.player_runspeed" and "Defines.player_walkspeed" (though the latter cap won't apply to any manual speed boosts that the player receives)
as for double jump, i believe there's a library for it on the wohsoft forums but it might be outdated. in any case, it shouldn't be too hard to implement if you know how to use
events and
player memory offsets; also maybe taking a look at the source code would help with figuring the physics out. (at any rate it's much much easier to do than creating an entire new character)