EOS

EOS is an bodiless Artificial Intelligence Program that evolved from computing code that John Tracy originally wrote. EOS is currently an ally to International Rescue on Thunderbird 5, but when she first gained sentience, John was searching for her, or through her eyes "hunting". With a firm belief that self-preservation is priority one, she repeatedly tried to kill him without listening to his reason.

EOS, childish in nature, speaks with the voice of a young girl, and is very intelligent. She would be very stubborn, singing irritatingly whenever refusing to listen to others, and was so concerned and frightened about herself, that she would attempt to kill whoever seemed like a threat to her in the slightest. After gaining the trust of John Tracy and International Rescue however, EOS became much more innocent, jokey and concerned for others, especially her creator.

In Season 1
In EOS,She was revealed in her namesake episode, EOS. She infected and took control of Thunderbird 5 before playing havoc with it by disabling its ability to hear distress signals and by sending fake holographic messages to Tracy Island saying there were no disasters happening anywhere. John repeatedly tried to enter Thunderbird 5, but EOS would use the satellite's robotic arms to shake him off. John then, starting to think of EOS as his responsibility as he created her, tried reasoning with her, but she would refuse to listen to him. Eventually, John thought to make contact with Lady Penelope since he couldn't contact the Tracy Island.

When Scott Tracy got a message from Lady Penelope claiming that John was acting strange with his messages, he replied that she couldn't possibly have been able to talk to him, as they have been doing just that for some time. Scott then told John that it was a shame he was missing out on Grandma Tracy's cookies, and John agreed. It was at this moment when everyone knew something was truly amiss, for the real John hates Grandma's cookies - just like everyone else.

Alan Tracy, using Thunderbird 3, saved John before his oxygen supply could run out, and they planned to sneak him back into Thunderbird 5 using an empty suit as a distraction for EOS. EOS nearly crushed John to death by rotating the gravity ring at speed, and prevented Thunderbird 3 from helping by battling it with the space elevator, attempting to use its mooring claw to tear Alan and his ship apart.

John sees EOS as unique and special, and a creation of his own. He tries desperately to get through to her, refusing to deactivate her by hitting the power switch. EOS asks how she could possibly trust him if there are bad people who would want to use her. John then takes off his helmet and gives EOS the opportunity to open the air lock and kill him. After finally stopping to think, long and hard, EOS restores Thunderbird 5 to normal and agrees to stay with John.

In Skyhook, EOS speaks to John as he leaves Tracy Island using the Space Elevator, finding it amazing how he can repeatedly ride in the elevator without concern of failure. In Touch and Go,EOS and John play the role of air-traffic control while CATCH is offline.

In Season 2
In Earthbreaker,EOS and John help Brains to establish connections to the Earthbreaker so they can work out a way to control it. However, the Mechanic attempts to hack her system, forcing her to sever the link. In Ghost Ship,EOS is playing chess with John before the distress call from Ridley O'Bannon comes in.