The Saika Army (AKA Slasher Army, Saikas, children of Saika) is a group of civilians who are possessed by the demonic sword, Saika. It got most of its members and power from the Slasher incidents in Ikebukuro. It's members mostly are not aware of the fact that they are possessed, and live normally, except when their "mother" calls out to them, or if they feel that something will greatly upset their mother unless they prevent it.
A person's eyes will glow red when they are currently under Saika's control. It is commonly represented by red, but is not a color gang.
The group was founded by Haruna Niekawa when she was the "Slasher." The cursed blade Saika has the ability to possess people it cuts, and Haruna was possessed by Saika, becoming one of its daughters. This meant that she had powers similar to Saika's owner, being able to hear the blade's desires and possess others using any knife she wields. Haruna believed she had suppressed Saika with the power of her love, but due to unfortunate incidents in her past the blade was able to take over. She became the "Slasher" for unclear reasons, but her blade fell in love with her teacher, Takashi Nasujima, and later with Shizuo Heiwajima.
Saika's love and desire to take over the human race led her to possess many people, and after one of Haruna's "daughter blades" met Shizuo, she and all her children went out on a rampage, slashing hundreds of people in Ikebukuro to try and find Shizuo. The rampage stopped when Anri Sonohara defeated Haruna using the original Saika, and possessed her, taking control of her and all her children.
Before Haruna's rampage ended however, hundreds of people were possessed, giving Saika's army enough members to be considered a leading force in Ikebukuro alongside the Dollars and the Yellow Scarves.
Due to a traumatic event in her past, Sonohara is able to control Saika, and thus does not use the army to take over humanity. She does use the power to help her friends though, and being the owner of the original Saika, she feels responsible for the Slasher incidents, and any consequences they have on the city. When the Yellow Scarves started a war with the Dollars on suspicions that the "Slasher" was one of them, Sonohara used possessed Yellow Scarves members to try and stop hits on Dollars.
A possessed Yellow Scarf also independently saved Masaomi Kida's life, stating that he could sense his "mother" approaching and knew she would be upset if Masaomi were to die.
The current leader is Anri Sonohara. Thanks to her ability to control Saika, and her control over all of Saika's children, the gang is largely an inactive and peaceful one, since Sonohara refrains from using her army for violence.
Recently, in the light novels, two rogue Children of Saika have started forming their own armies for various purposes. Haruna Niekawa manages to break free of Anri's influence once again and uses her own children to supplement Izaya's forces. Outside of the women from the group "Amphisbaena", there are no named members under Haruna's command. In volume 10, when Izaya Orihara set Shizuo Heiwajima up for arrest, the two police officers interrogating him were under Saika's influence. They were almost definitely controlled by Haruna, although there is a slight possibility that it was done by Kujiragi, as she states that Shizuo is an "obstacle".
The second rogue is Kasane who is Jinnai Yodogiri's secretary. She claims to use another form of creating children other than cutting, namely "branching" (breaking Saika's blade and smelting new blades from it), that Anri most likely does not know of. The only known person under her control as of volume 10 is Slon.
Thanks to the undetectable nature of Saika's influence, the Army has members throughout the general public, as well as within other gangs such as the Yellow Scarves.