The Keys to Lyoko

History
Previously, the Keys to Lyoko were held only by Aelita and Franz Hopper. XANA eventually stole them from Aelita's memory in the episode, "The Key." When XANA executed his back-up plan, he lost the Keys. The Keys cannot be copied like normal files, thus the loss.

How They Work Now
When Franz scanned the Lyoko Warriors to rid them of their Lyoko DNA, it also removed the Keys from Aelita. The Keys were then stored in the Core of Lyoko. Over time, they took the form of "Info Orbs."

When a human possess the Keys to Lyoko, it changes the color of their eyes to be their Lyoko Color. This only happens when the Key Holder is thinking about Lyoko-related subjects. Their eyes also glow, enough so that they could be seen in the dark.

They Keys allow Humans to deactivate Towers and get into Replikas. Also, something special happens with the Keys. This has not been revealed yet.