Kefka as a playable character?

Everything you need to create such a hack is available on this site, be it tools or reference. You just need to apply the time and effort. But yes if you have questions along the way please ask.

Really,how do you guys can work in this? any tutorial or something?

Use the search feature dude. What exactly are you planning to start on?

I don't know,where do u recommend I may start?

It depends on the scope and goal of what you want the hack to have, and how it will differentiate from the original, besides just swapping one of the characters with Kefka. So I guess start by planning out the hack and what exactly you would like to do with it. Then you would put the Kefka sprite in and maybe learn a bit about event editing so you could edit and create new scenes with him. Or you could just leave it at a character swap and give him dialog and call it a day, but that'd be boring.

And once you're ready, if you'd like to and are serious about creating this hack, you could make a thread in the personal projects forum and use that as a place to ask questions and post details about it and all that.

Ok,i'll make another question,asking the people that if there was a Kefka hack,what would they like,please vote yourself

Ok, so, I've actually been planning out a hack that still keeps within the original FF6 story, for the most part, but changes some things around.

1 - Kefka is the main 'hero'. I use that term lightly. Before he could enslave Terra, he was sent out on the mission to retrieve Narshe's Esper.

2. Terra is a cold hearted, no mercy kind of villain. She was raised from birth by the evil Emperor Gestahl. She would be ruthless, cold, calculating. The main reason she's not, in FF6, is because of her amnesia.

3. While the story remains pretty much the same, a lot of the dialogue is changed, such as Kefka constantly trying to fight back against the Narshe guards, but unfortunate events keep happening. Falling down a hole, for example.

4. No one in the party likes, or trusts, Kefka. Ever. He's a sadistic, evil clown, who's only working with the party in the hopes of overthrowing the Empire, and taking it over for himself. The party is only working with him, because he's one of the highest ranking officials. He's very powerful.

5. Speaking of power, if there was some way, I would like to include very powerful magic for Kefka at the beginning of the game. Fire 3, Meteor, stuff like that. All offensive, no curative or support of course. But Tritoch drains his power, forcing Kefka to be weaker, and regain his power. Sorta like Jade, from Tales of the Abyss, if anyone played that.