Very little is actually known about Necro. He was originally a normal boy from Russia named Illia, who was kidnapped and experimented on by the Illuminati's "G-Project". The project that mutated him into Necro was the experiment that preceded the project to create T.W.E.L.V.E. Despite the torture and pain inflicted upon him during the mutation, he kept his good nature and humanity, and fought only for his survival and freedom.
While captured by the Illuminati, he met a girl named Effie, and they developed feelings for one another. Necro entered the third World Warrior tournament for a chance to fight and beat Gill for his freedom, but he was caught. However, his girlfriend Effie managed to save him, and they are now fugitives together. Effie is very devoted to Necro, and often mimics his actions at the end of each fight.
Due to the mutations inflicted upon Necro, he can stretch hims limbs in a manner very reminiscent of Dhalsim. However, he also has the ability to conduct electricity through his body much like Blanka. He is slow, but can almost be used with equal effectiveness up close and far away.
When you perform Necro's personal action (+), his attack power increases for all of his moves except throws (this includes his Snake Fang, German Suplex, and Slam Dance). His personal action can hit multiple times, and you can hold down HP+HK to make him keep flailing his tongue at you.