Short answers, pulled from the story.
Akira Yasuda created a paper clay model for the sprite artist to reference during development. Producer Tomoshi Sadamoto oversaw Necro and Oro's creation directly because he doubted how audiences would receive their unusual designs.
The young Russian man known as Illia originated from a lakeside village after the fall of the Soviet Union. He traveled to find fame and fortune but was tricked by Gill's Illuminati into having his DNA altered to become a living bioweapon with an elastic body and built-in power generator.
Necro stands 174 cm tall with a hunched over body, long arms, and white skin except for red sun markings on his shoulders. His outfit consists of purple rubber pants with a matching harness, large metal braces around his neck held by bolts, and a pierced tongue that creators wanted to emphasize.
His attacks put him at a significant disadvantage due to reliance on extending limbs which conflicted with the defensive nature of the game. Players could parry attacks easily making Necro players hit in retaliation more often than not according to analysis by Shivam Bhatt and Host Diamond Feit.
Necro was originally voiced by Michael X. Sommers until Lawrence Bayne took over the role in Street Fighter III: 3rd Strike. The voice change occurred after the initial release when the character faced criticism regarding his performance style.