TEC-XX (or TEC, as he likes to be called) is the main computer for the X-Naut labs, all of which features in the game Paper Mario: The Thousand Year Door. Built by the X-Naut leader, Sir Grodus, for his own purposes, TEC-XX claims himself to be "The worlds best and only perfect computer". What the X-Naut labs use TEC for is unclear, but TEC has several "security cameras" placed around the X-Naut fortress that are directly connected to his systems, so he can monitor the entire fortress with ease.

TEC's true purpose of existence, the one Sir Grodus built him for, is blurred to TEC when he meets Princess Peach. Captured by the X-Nauts and taken to their fortress, Princess Peach is brought before Sir Grodus and is questioned. TEC watches the "interrogation" from one of his security cameras, but something odd happens to him. From what TEC can determine, Princess Peach's image had caused him to experience processor deviations, his CPU to overheat and cause malfunctions to his internal systems when he watched her. Unsure of what to do, TEC-XX leads Princess Peach, who was sent to the holding cell, into his room to find out what is wrong with him. Not only that, but he also wants "to observe her" and "to learn more about her". After a small intoduction, TEC tells the princess about what he experienced whilst wacthing her, and also why he led her into his room.

After hearing all this, Princess Peach comes to the conclusion that TEC is in love with her. Being a computer with insufficient emotional programming, TEC does not know or understand what love is. To fulfill one of his purposes as "the worlds best and only perfect computer," TEC asks for the princess's help to teach him the meaning of love. In exchange, TEC allows Princess Peach to communicate with Mario via e-mail to let him know that she is safe and tell him how she feels. Later on, she also gives Mario intel as to what the X-Nauts are planning to do, through the assistance of TEC.

TEC is also a clone of HAL from 2001: A Space Odyssey; only TEC is green and was originally built for evil purposes and turned good. Instead of being red and built for research and turned evil.