SOIS banner with an eagle and star.
According to the artbook, their banner is purple.
The Special Office of Internal Security, also known as the SOIS, is a secret organization which Clover, Light, Nona, Ennea, and Alice belong to, as well as all of the participants from the First Nonary Game, except for Aoi and Akane Kurashiki who are Crash Keys members.
The SOIS is under the jurisdiction of the Department of Defense. They investigate potentially disruptive or dangerous elements, such as domestic or foreign terrorists, radical political splinter groups, and religious organizations with extreme agendas that could pose a threat to the state or the citizenry. Their stated purpose is to maintain peace, but their charter authorizes them to arrest, detain, or even kill. The SOIS' existence hasn't been made public, so there are only a few people who even know they exist.
Alice and Clover, two members of the SOIS.
Clover and Alice's roles involve comprehensive investigation on criminal and terrorist organizations (i.e. Free the Soul). To deter suspicion away from being members, they do not wear uniforms, and instead wear casual clothing, as the existence of this organization has not been made public yet. There are rumors that some of the members of the organization would dress up like Elvis or Lady Gaga, and even some agents dressed up like Captain America.
The very few people that know of the organization are said to be teasing them by calling them the "State Of InsanitieS".
It is unknown what SOIS' role is after Zero Time Dilemma, but it is possible SOIS may join with Crash Keys to defeat the religious fanatic who Delta spoke of.