well first you have to find out if the character you edited is open source, sprite or coding and if its not open source youll have to ask for permission to the author for release it, also if you took coding or sprite editing from other creations youll have to ask first and credit them too.
now if the character is open source and you did most or all the edited parts there will be no problem on releasing it.
BTW you can check the open/not open source creations topic at mugen discussion, or ask there.
D-Day