Alright then.
A clone character is a character that resembles another already established character, whether it is in appearance or gameplay… or both.
It’s not necessarily a direct copy, but have obvious resemblances to the other character.
For example, Ken is a rather obvious clone of Ryu, both in looks and gameplay. However, he has evolved some unique tech to his moves and is slowly moving away from looking like a “red Ryu with blonde hair” by adding some other clothes to his main look, such as sporting a t-shirt and with his gi-shirt hanging from his belt. Stuff like that.
Then there’s a character like Sakura whose moveset is obviously inspired by Ryu without looking exactly like it, and she even fashions herself with a headband just like Ryu, though she wears a schoolgirl’s outfit.
And then there’s Sagat who doesn’t look like Ryu at all, but he has a fireball and an uppercut that resembles Ryu’s. In SF lore, it’s because Ryu gave Sagat his scar with a Dragon Punch, and that’s why Sagat has tried to copy it. How much of a clone character he is though is debatable, as it is basically just his uppercut that is a copied move. Because though he does have fireballs, they are somewhat different. He would indeed be on the far end of the clone character spectrum though.
Some smart-asses would say “He punches and kicks, and this character also punches and kicks, then he must be a clone character!”, but no… unless those punches and kicks animates just like the other character, then he wouldn’t be a clone character.
It also covers alternate versions of a character, such as Evil Ryu or Shadow Jago.
This is why Kilgore is a clone character. He shares alot of moves with Fulgore, most of his normals being directly taken from Fulgore, and his design resembles Fulgore’s as well, due to him being a prototype version of him. However, because of his chaingun arms, his tech is different as well, adding the bullets to some of his special attacks. Sure, he has some unique features and special moves, but there are enough resemblances to Fulgore that makes him a clone character.