A lot of fighting game franchises have two specific characters that are so iconic that you just can’t have those games without them. Street Fighter has Ryu & Ken, Mortal Kombat has Scorpion & Sub-Zero, Tekken has Kazuya & Jin, KOF has Kyo & Iori, and Guilty Gear has Sol & Ky.
Although, in the case of King of Fighters, I guess the same could be said about Terry, even though he's originally from Fatal Fury.And I think the examples for Killer Instinct may be Jago & Fulgore.