Kazuya Makigami is the older brother of Naoya Makigami, and physically abuses him every day to vent his stress over his criminal life.
Despite his age, Kazuya is actually the leader of a band of criminals who rob restaurants in Shibuya. The gang refers to him as "Makki", and he acts as their lockpicker. He eventually becomes fed up with the way his underlings treat him, and discovering the Phantom Aficionado Website, almost posts on there to have them killed, but decides against it. When two of his underlings are laid off due to "that geezer of a painter", he tells them not to go overboard.
Kazuya goes to rob Cafe Leblanc with his gang, but is surprised when he learns that it is in Yongen-Jaya. After breaking in, he becomes distressed when he notices a calling card from the Phantom Thieves of Hearts with his name on it. When the heist goes bad, he wonders who could have ratted them out, and finds himself in the Palace, where the Phantom Thieves confront him. There, they reveal that Naoya posted about him and his abuse, and he snaps, revealing himself to be the gang's leader and that he believes that everyone is inferior to him. He transforms into Mithras, and summons an Oni and Onmoraki to fight the Thieves. He is ultimately defeated by them, with Joker finishing Mithras off with a bullet to the head, having "stolen" the arrogance from his heart. Defeated, Kazuya gives the key to his desk drawer to the Thieves, instructing them to give it to Naoya and have him burn what's inside.
In the real world, Kazuya loudly admits to his and his gang's crimes in public, leading to their arrest. He notices the protagonist walking away, wondering who he is.
Even though in The Day Breakers anime adaptation the Phantom Thieves decide to use Leblanc as bait to lure Kazuya and his gang this same plan in the game was brought up but denied right away due to the troubles that may bring to the protagonist.