The Black Siblings are four orphaned, teenage siblings who go by the name of Bachika and claim to be Beastmen-hunters. The eldest is Kittan, followed by Kiyoh, Kinon and then Kiyal . In the first half of the series, all four siblings are clad in black outfits, hence their group's name. In the second half of the series, the three sisters wear normal clothes while Kittan, being a government official, wears his uniform.
Before Beastmen War Arc
Some time before the events of the show, the Black Siblings fled their village after its destruction. The siblings then decided to rid the surface of Beastmen by hunting them with oil bombs that they had either found or crafted.
Beastmen War Arc
When Simon and Kamina are training (which involves Kamina throwing boulders at Simon), the Black Siblings mistakenly believe that Simon is being attacked by a Beastman's Gunman and rush to his aid. Kamina steps out of Gurren after it is immobilized by the siblings' oil bombs, leading to the two groups meeting in-person.
The three sisters then proceed to go hunting and find a strange, furry creature. When they take it back to be eaten, the creature turns out to be 16 small Beastmen. The beastmen attacks the gang in its Gunman and the three sisters are thought to have been killed when they are buried under debris from the fight. The three are soon revealed to be alive, and head off with Kittan in a different direction from Simon and Kamina.
Some time later, the sisters appear at the bathhouse, explaining they became lost and separated from Kittan whilst traveling. When the bathhouse turns out to be a Gunman, Yoko frees them and they mange to escape with Kittan. Viral attacks the two groups, but gives the siblings and Team Dai-Gurren time to dress themselves. Kamina and Viral battle when the rest of the gang is attacked by Gunmen. Kittan saved them in his King Kittan and they watch Gurren-Lagann take down Dai-Gunzan.
When the rest of the group goes off to fight, they stay behind to keep the place safe. They soon learn of Kamina's death and become depressed, except for Kiyoh, who decides to pilot a Gunmen. Before she takes over as a Gunman pilot, she works in the bottom command room of Dai-Gurren before it is destroyed by a Beastman's Gunman. Kiyoh soon takes the Dayakkaiser and goes out to help the rest of the team's pilots. During the battle of Teppelin, Kiyoh has the Dayakkaiser join Kittan's King Kittan and Kiyal's Kiyalunga in order to fight the main army while Simon deals with Lordgenome. After his death, the siblings appear with the others, cheering for Simon.
Before Anti-Spiral War Arc
Sometime before the second part of the series, Kiyoh married Dayakka and soon after becomes pregnant with the child that would become the 1,000,000th person to be found on the Earth's surface. Meanwhile, Kittan and Kinon are given positions in the new government instituted by Team Dai-Gurren.
In the Manga
In the manga, the Black Siblings appear earlier in the story (during Team Gurren's time in Littner Village) and explains to Simon that their home village, Bachika, was destroyed by Viral.