A-21 is a small terrorist cell focusing operations in Japan. They are funded by Amalgam and become the focus of Mithril operations when they personally assaulted a secret Mithril meeting in which TDD-1 Captain Testarossa was present to observe and gather intel on a boy who was arrested for assaulting an airport personnel. The captured boy was showing signs of being administred with a drug ("TI-971") used to help control a Lambda Driver equipped AS. A-21 sacrifices most of its leadership and assets when they attack Tokyo with the Behemoth and afterwards cease to become a threat.
A-21 was initially a welfare organization funded by a former mercenary, Seiji Takechi. It's purpose was to rehabilitate juvenile delinquents who had gotten into considerable amounts of trouble. He gathered these children and sent them to a deserted island he had purchased. Takechi taught them complete survival and battle skills. They learned how to survive and win a battle where electricity, water or food were not available. As Seina explained:
"We gained a fortune called confidence. We were so happy when we realized we didn't need to commit crimes any more. But other people didn't think so. One time, a TV station found out about our training. They landed on the island without permission. They touched the equipment and there was an accident. It was such a mess after that. They all accused us no matter what caused it. They called A-21 a terrorist group and had the training facility shut down. They revealed the past of the students. Mine too.
We want to paint this town full of peace in our colours. If you call that revenge, you're right. Destroy it completely and cover it with horrifying flames.