K9999 is a clone from Kyo Kusanagi's DNA.
Due to his (apparently too close) resemblance to Tetsuo Shima (even sharing the same voice actor), he was replaced by Nameless in The King of Fighters 2002: Unlimited Match. He, however, remains in the unaltered version of KoF 2002 that is included with Unlimited Match.
In KOF XV, K9999 returns after a long absence, but has changed his appearance and taken on the name Khronen McDougall to evade surveillance from what's left of NESTS. He leads a team in the latest tournament with Angel and Kula as his allies.