Why Alice?

When Gabriel and his men infiltrated the Ocean Turtle and dove into the Underworld, they were after a special person: Alice. She is a true bottom-up Artificial Intelligence (AI) with the accomplishment of overcoming the restrictions placed upon her. Since the AIs of the Underworld are made from copied fluctlights, they are essentially human in all ways but physically. A large issue for them, though, is the Code 871, otherwise known as the Seal of the Right Eye. The humans of the Human Empire must never defy the Pontifex, the Axiom Church, and the Taboo Index. In the Dark Territory, the inhabitants must obey the dark god Vector without question. If either side goes against this inhibitor, the right eye of the person will explode. Underworlders experience great pain in the right eye that usually keeps them from actually disobeying the law, but a few of them have broken through the seal. Along with a few Dark Territory leaders, Alice and Eugeo were both able to do so.

Although Alice is not the only person to break through the seal, she is still set apart from the rest. At 11 years old, Alice, Eugeo, and Kirito all traveled to the End Mountains to gather ice. They got lost on the way out, and they walked out just in front of the Dark Territory. Crossing into it would mean violating the Taboo Index. They witnessed an Integrity Knight fighting with a Dark Knight, both aboard a dragon. The Dark Knight was struck down to the ground near Alice and the others. When she saw this knight pleading for help, Alice ran towards him and tripped, causing the tips of her fingers to cross into the Dark Territory. The following day, she was arrested by Deusolbert for her crime and was taken to the Central Cathedral in Centoria. This is how she became an Integrity Knight two years later.

When Alice violated the Taboo Index, she did not face an issue from the Seal of the Right Eye. Her heart was filled with pure goodness and compassion for the Dark Knight, even if he was considered the enemy. When Eugeo and the two Dark Territory leaders broke their seals, they had to allow their right eyes to explode. Alice did not break her seal until she and Kirito were forced to climb up the Central Cathedral, where she learned the truth about her past and the memories that had been wiped away from her. She needed to break the seal to muster the resolve to fight against the Pontifex (aka Administrator) and the Axiom Church itself. She learned the truth about Administrator's lies and decided to fight for what she felt was right, alongside Kirito.

Up to the point of the invasion of the Ocean Turtle, Alice was the only one in the Underworld with a broken seal. Eugeo lost his life in the battle with Administrator, and the war with the Dark Territory had not yet broken out. Every time a person from the Underworld broke their seal, they were under the influence of a person from the real world. They were able to fight against the system that bound them.

Alice is special to the invaders because, as a bottom up AI, she went against the rules of the system. She was able to use her own judgement and think for herself, even after being raised in a virtual simulated civilization with the law upheld almost obsessively. The enemy wanted to use her for warfare, but Kirito and the others chose to protect her and the Underworld. To them, she was no different from a human in the real world. At that point, there was no real difference. Alice was human.