TEC-XX, better known as TEC, is the main computer of the X-Naut Fortress during the events of Paper Mario: The Thousand-Year Door. TEC was designed to be "flawless and perfect", though a sort of glitch resulted in him apparently feeling love, which led to him ultimately revolting against the X-Nauts. After each chapter, he talks to Peach in an intermission.
When Peach is first kidnapped by the X-Nauts and held in the X-Naut Fortress, two doors leading to TEC automatically open for her. When she meets TEC, he first praises Grodus, then explains that he had a special reaction to Peach and wants to "observe" and learn more about her. Peach finds this weird and questions if TEC is in love with her; he reveals that he cannot comprehend the idea of love and is incapable of feeling it. Wanting to be the world's most perfect computer, TEC asks Peach to help him feel love, offering to grant her wishes in exchange. Peach asks to be able to contact Mario, and TEC allows her to do so on her keyboard, sending a message to Mario's Mailbox SP. He states that he does not have any need for Peach left at the moment and allows her to leave.
After chapter 2, Peach is heard in her room, wondering if she will ever be able to dance in the Mushroom Ball again. TEC calls Peach into her room and assures her that Mario has received the email, then admits to eavesdropping on her. TEC explains that, having heard Peach, he now wants to dance with her, and creates a holographic clone of Peach for her to dance with. After the dance, TEC realizes that he had fun doing it, and lets Peach send another email to Mario before sending her back to her room.
During the intermission after chapter 3, TEC again calls Peach over. This time, he asks her to question Grodus on an issue. TEC opens the elevator and instructs Peach to wear an X-Naut uniform, then talk to Grodus. While in disguise, Peach asks what Grodus plans to do with her, and he responds to keep her in her room but not harm her, although refusing to say why she is needed. Grodus unknowingly tells Peach about their plan to conquer the world. Upset at TEC, Peach returns and asks why he intends to take over the world. TEC asks Peach if she likes the world she lives in, and she answers that she does. TEC refuses to tell Peach about how the Crystal Stars are involved in the plan, but allows her to send Mario a message about it.
Peach returns to TEC after chapter 4. TEC is quiet and asks if it is more important to carry out Grodus's order or continue keeping Peach safe. In order for her to send Mario a message, TEC asks her five questions about Grodus's plan. After doing so, she is shocked but continues to send a message, then goes back to her room.
While initially angry at TEC, Peach goes to his room after chapter 5. TEC requests a data disk from Grodus's office, and makes Peach go to a laboratory and mix an invisible potion so she can sneak into the room. When Peach changes the disks, TEC is allowed to view info from other computers, and analyzes the data. Saying that he will tell Peach when the data is figured out, she lets him send Mario an email and then tells her to leave.
After chapter 6, TEC has finished analyzing the data and calls Peach into his room. He explains Grodus's plan, and realizes that it is more important to ensure Peach's safety than follow Grodus. TEC says that Peach must escape as fast as possible, telling her that she is on the Moon. As Peach writes an email to Mario before escaping, Grodus enters with two X-Nauts and calls TEC a traitor. The X-Nauts shut off TEC and delete his data, to Peach's horror.
While exploring the X-Naut Fortress in chapter 7, Mario comes across TEC on the bottom floor. Running on backup power, TEC tells Mario that Peach has been taken to the Palace of Shadow with Grodus. He opens the door to the teleporter leading to Rogueport Sewers, instructing Mario to save Peach and give her his thanks, before shutting off.
In an email sent to Mario by Goombella in the game's ending, it is revealed that TEC is still alive. Goombella says that TEC really wants to see Mario and Peach again.
In the Peach intermission after chapter 4, TEC is in a conflicted mood and asks Peach to have a quiz. If Peach answers a question incorrectly, TEC will restart the quiz.
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