Unlike the Innocent Sin Shadow Selves, Shadow Katsuya was created by Nyarlathotep to taunt Katsuya and make him feel miserable. He uses the Persona ReverseHyperion.
He appears in the Room of Reason and taunts Katsuya that he still regrets of not following his dreams. Katsuya denies this but the Shadow says that lying is the start of evil, he could have had his way if it wasn't for his brother and father and because of it Katsuya despises them both. The Shadow says that Katsuya must be tired of playing the good older brother and that his love for the family is just a role. Katsuya still tries to deny it but he is unable to do so. The Shadow tells Katsuya that he must stop playing innocent... that he is in love with Maya. Katsuya is just deceiving himself in front of his brother, and that he must not protect such a brother any longer. The Shadow kept taunting Katsuya until Katsuya turned to Tatsuya and pointed him with his gun, Ulala begged him to stop and then asked Maya to do it. Whichever option the player chooses, Katsuya will either act weak or he will fight his own demons, but either way he will accept what the Shadow said. If he acts weak, he will apologize to Maya and Tatsuya and say that he sometimes detested him and his father. If he decides to fight his own demons, he shoots the Shadow telling him that he made a mistake in getting between him and Tatsuya with that face and tells him to get lost. Depending on the choice the player makes, the difficulty of the fight against Shadow Katsuya will be harder or easier.
The party arrives at the World of Judgement and they see Eikichi. Tatsuya tries to set him free but Shadow Katsuya and Shadow Ulala prevent him from doing so. The difficulty of this boss battle depends on the choices made in the Room of Reason and the Room of Instinct.
After the battle ends the Shadows disappear and Eikichi is set free. Philemon appears and tells Tatsuya that he will send Eikichi to the real world and that he knows that Tatsuya wants to take Eikichi with him but that is what Nyarlathotep wants. After he disappears with Eikichi, Tatsuya swore to protect "This Side" and everyone else.
Shadow Katsuya voids fire, nuclear and dark spells so use ice or water spells as those are his weaknesses. Make Maya use Mediarama with whatever Persona she has or Diarahan with Artemis because both Shadows deal a good amount of damage in both difficulties. Make Tatsuya use Gigantic Fist with Apollo against Shadow Katsuya as that is one of his attacks that isn't fire based and is stronger than his normal attacks. This Shadow most dangerous attack is Vice Shot that has chances to instantly kill a character, so have a Persona with a reviving spell or numerous reviving items. Shadow Katsuya's Shadow Force will make the player unavailable to see the HP and SP of the party and not be able to enter the status menu, this isn't critical since it will wear off by time but be sure to heal often and remember what kind of status spells the Shadows use so you can heal it.
In the case of Shadow Ulala, she reflects mind and nerve spells, voids wind and dark spells, and is weak against earth spells. Use the same healing strategy with Maya and attack paterns of Tatsuya. Baofu can use Magdyne, an earth based spell that is gained after a mutation, against the Shadow. The Shadow may use Alluring Mazurka that will charm all characters, use items to revert it. Katsuya and Ulala can attack with different Personas or just use regular attacks against both Shadows.
If all characters have their Ultimate Personas with the necessary skills, they can use the Fusion Spell Dragon Cross that can cause over 2000 HP of damage to both Shadows.