Ehh... no one.

Please don't mix Zelda with MK. Because that would open the gates of Hell itself and bring forth the repture.LOL
Some people here tried to suggest the best characters to fit into MK, it wasn't pretty. Personally I can see Ryu Hayabusa in it as a console specific character only. Others say Kain and Raziel from Soul Reaver, but they're thousands of years in the future so that could never work. All I'm saying is MK has made arealm that makes it impossible for outside characters to blend in. I think it shoulod stay that way.

But how do you explain Spawn in Soul Calibur 2????????

Spawn's cameo in Soul Calibur is an exception. Not only does he fit in it.Look at some of the other characters, a dead pirate, a possed knight, a woman with a living sword and a gollem hell-bent on destruction. But it was also a business by-product. Todd McFarlen was asked by Namco to create a new character for Soul Calibur, so Necrid was the result, which I think is a great character, don't know why people are ragging on him. And the Xbox got Spawn as a result, which I think makes the most sense. You should be asking, how the hell Link got into Soul Calibur II, THAT makes no sense.