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 apparently designed to be "flawless and perfect", though a sort of glitch resulted in him (despite his being mechanical) being apparently able to feel love and ultimately revolting against the X-Nauts.
TEC is typically seen after Mario and his partners complete a chapter; more specifically, TEC is most often glimpsed during scenes where Princess Peach is controlled. As stated above, a strange glitch gave TEC a crush on the princess, although TEC himself could not fully understand love. This particularly annoyed TEC, who had apparently been told earlier that he was designed to fully understand everything known to the universe. Peach was also one of TEC's only companions and therefore the only person who could possibly teach him about love, and TEC saw an opportunity in this: Peach greatly sought TEC's ability to e-mail anyone (which was her only way to contact Mario, who had armed himself with a Mailbox SP sometime before the events of Paper Mario: The Thousand-Year Door), so TEC offered her to use this e-mail feature in return for lessons about love. He would also often request that Peach go undercover and retrieve secret information from Sir Grodus, his master, for he wished the know the X-Nauts' secret plans as to reveal them to the heroes. (This was because his apparent love for Princess Peach had slowly converted him to her side.)
By the end of Chapter 6, TEC (thanks to Peach's escapades) manages to decipher what is on Grodus's data disk and completely understands his plot. TEC reveals exactly why Peach is being held captive (which was actually so the X-Nauts could let the Shadow Queen possess her later on), but just as Peach is typing an e-mail to Mario, Grodus unexpectedly bursts into the room. Peach and TEC are both in shock as Grodus, stating his awareness of TEC being a traitor, orders two of his X-Nauts to press the red buttons on TEC's sides at the same time, permanently shutting him down. The X-Nauts succeed in their plans, and Peach was taken off to unknown parts, but it was revealed that TEC had apparently learned of love and its acquisitions at this point, for he said to Peach "I love you" as his last words.
The last scene involving TEC occurs during Mario's infiltration in the X-Naut Fortress, specifically right after Mario has destroyed Magnus von Grapple 2.0 and collected the last Crystal Star. Here, he locks all of the doors except the one leading to his own chamber (which had previously been unavailable), eventually leading Mario and his partners to the room he is in. He then tells the gang that he is running on backup power and cached memory, and, while such rations allow him to survive, they are running out and affecting the quality of his operations. (Indeed, TEC speaks in a strange, pause-filled sequence in this form and seems immensely weak, although he is still able to remember Peach and his apparent love for her, even being able to produce an image of the princess standing in her room.) After explaining a few things to Mario and telling him that Grodus has taken Peach to the Palace of Shadow, TEC releases the room lock in the Fortress and commands Mario and friends to save Peach, giving them the specific order to tell her "thank you" on his behalf. He then reveals that, in order to let Mario and friends escape the X-Naut Fortress, he will be forced to send all of the AI energy programmed into himself to the teleporter room (presumably because the X-Nauts had somehow disabled it in hopes of trapping Mario), causing him to "cease to exist as an artificial consciousness". This, however, is a price he was willing to pay for Peach, and he ultimately shuts down after his speech as an unknown voice reports a "TEC system failure". TEC's final words are "Peach... Goodbye...", before a great explosion engulfs the Moon and he is seemingly destroyed. However, at the very end of the adventure, TEC reboots and the X-Naut Fortress is up and running again, although the whole place is deserted. (This is because it was apparently TEC himself who detonated the Fortress as to remove all traces of the X-Nauts, though he seemingly destroyed himself in the process.) He seems to have been restored to his original state after Peach gave Mario the last of her power during the battle with the Shadow Queen, which is implied by the fact that he claimed to have seen a bright light and heard Peach's voice as he awoke, though he himself did not know how he had survived.
In the Peach intermission between Chapters 4 and 5, TEC is in a conflicted mood and will ask Peach to have a quiz.
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