This is a character that lets you play the first level of Super Mario Bros. It is the full level that you can play as any character to defeat the goombas and stuff(thanks xeno for the correction) You must select the character as player 2.
Place the files m-nes_stage1.def and m-nes_stage1.sff in the stages/ directory of your MUGEN folder. You should be fighting against this character in the stage that is included in the file, the character will ONLY work in this stage and it is preferred that you choose a small character.
You must select the character as player 2. 
i thought i included the stage in the link as well, i updated the link to include the stage