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 his X-Naut masters.
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) seemed to have developed a complete understanding of Grodus's plot. TEC was just about to reveal exactly why Peach was being held captive (which was actually so the X-Nauts could let the Shadow Queen possess her later on) to her when Grodus himself unexpectedly burst into the room. Peach and TEC were both seemingly horrified as Grodus, stating his awareness of TEC being a traitor, ordered 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 succeeded 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 the heroic plumber had destroyed Magnus von Grapple 2.0 and collected the last Crystal Star. Here, he locked 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 was in. He then told the gang that he was running on backup power and cached memory, and, while such rations allowed him to survive, they were running out and affecting the quality of his operations. (Indeed, TEC spoke in a strange, pause-filled sequence in this form and seemed immensely weak, although he was 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 had taken Peach to the Palace of Shadow, TEC released the room lock in the Fortress and commanded Mario and friends to save Peach, giving them the specific order to tell her "thank you" on his behalf. He then revealed that, in order to let Mario and friends escape the X-Naut Fortress, he would 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, was a price he was willing to pay for Peach, and he ultimately shut down after his speech as an unknown voice reported a "TEC system failure." TEC's final words were "Peach... Goodbye...", before a great explosion engulfed the Moon and he was 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 inferred 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.