Kingdom Hearts: Chain of Memories
Lexaeus first appears in Kingdom Hearts Chain of Memories during Riku's storyline. In Chain of Memories, a large portion of the story was the struggle occurring within the Organization itself, a struggle that Lexaeus was heavily involved in. When Larxene and Marluxia begin to show signs of dissent, Zexion and Lexaeus begin to monitor Castle Oblivion and their activities. Axel and Vexen are two other members who were active within the castle. Axel is considered to unpredictable to trust, but Vexen is enlisted by Zexion and Lexaeus to help monitor Marluxia. After they catch wind of Marluxia's plan to manipulate Sora and use him to overthrow the Organization, Lexaeus, Zexion and Vexen decide to use Riku to counter Sora.
Vexen is first to act, and tries to make use of Riku's power by gathering data on him and creating a replica (aptly named 'Riku Replica') but his experiment is interrupted. He is successful in creating the replica, but is killed by Axel of Marluxia's behalf before he can take control over it (the replica would then become independent and search for a heart, or a way to make himself 'real')
Lexaeus then takes matters into his own hands, as he confronts Riku shortly before Sora confronts Axel and Marluxia at the top floor of Castle Oblivion. He tries to manipulate Riku into giving into the darkness within himself and use his dark powers. Riku refuses and the two battle, with Lexaeus figuring that if he cannot be manipulated, he must be destroyed. After an intense and close battle, Lexaeus is defeated, but claims that the Organization will triumph. He uses his last power to turn the area into darkness, hoping for Riku to embrace it and give into it (which he did not, although it did allow for another confrontation between Riku and Xehanort's Heartless).
Kingdom Hearts II Final Mix
Lexaeus returns as a secret boss in Kingdom Hearts II Final Mix along with all the other Organization members that were defeated in Chain of Memories. In the battle, Lexaeus and Sora each have a power meter that Lexaeus can increase. When he does so, a large red/pink/yellow aura powers up around him, which protects him from flinching, minimizes damage dealt to him and pushes enemies away. He is exceptionally strong and will throw Sora up into the air and then knock him back down or knock him into a pillar and then slam him with his giant Tomahawk. He often integrates the element of Earth into his Tomahawk attacks, which increase in damage with his power meter. With almost every one of his attacks, rock collumns rise out of the ground (which does not occur in Chain of Memories, although he would occasionally heave rocks out of the ground in that game). Sora's reaction command allows him to draw power away from Lexaeus onto himself.
Kingdom Hearts 358/2 Days
Series Director Tetsuya Nomura confirmed that Lexaeus would be a playable character in the multiplayer mode for the DS title Kingdom Hearts 358/2 Days, and stated that he will "have slow movement, but will be very powerful".