Mukai is one of Those from the Past, who is trying to awaken Orochi so that his master can absorb it's powers. His origins remain a mystery. He decides to use the King of Fighters tournament to gather energy for Orochi, but since he didn't wanted to call attention, he decided to manipulate the tournament indirectly. He orders his subordinate Botan to manipulate Chizuru Kagura and Rose Bernstein to host the tournament, while he tries to break the seal of Orochi. Mukai is amazed to see the prowess of the fighters of the tournament, admiring their vast potential. As he breaks the seal, he watches as Chizuru Kagura is defeated by the winning team, deciding to show himself and challenge the team. He is defeated, but he then stands up as if nothing had happened, and disappears, leaving Shion and Magaki to take on from where he left.
Mukai appears again in The King of Fighters XIII, there, he is following Saiki, who is revealed to be his master, but Saiki is upset with his subortinates successive failures. At the end of the tournament, Saiki freezes time and orders his underlings to gather their powers to open the Gate of Fate, but Mukai refuses, as he wants to fight the winners again to exact his revenge. Displeased, Saiki hangs Mukai by his neck and drain his vitality, killing him.
Mukai is a very proud warrior, who never underestimates his adversaries. He loves to talk in grandiose declarations, as if he is a prophet.