Unlike in Japan and the rest of Earth, where slavery is considered a heinous violation of human rights and a crime against humanity, and is practically universally illegal, the Special Region has no such views, or indeed, any real concept of universal human rights, and slavery is commonplace.
Much like its real-world basis, the Roman Empire, slavery is legal and commonplace in the Empire, with most people sold into slavery being captured prisoners of war. This practice has apparently become so common that the Empire has been known to declare war on weaker nations simply for the purposes capturing slaves, as happened during their attack on the Warrior Bunny Tribes. While many people taken into slavery in the Empire are Humanoids, this is not universally the case- members of any race that are captured in battle can be taken into slavery. Slaves of the Warrior Bunny tribes are marked by having the end of their ears cut off. It is not know if this practice extends to other races. Enslaved peoples in the Empire typically end up used either for dangerous forced labor, often in mining, while female slaves are often used as sex slaves, as happened to Tyuule and Noriko Mochizuki, both of whom were bought by Zorzal El Caesar along with several other women.
The discovery that a few Japanese citizens were taken as slaves led to the breakdown of peace talks between Japan and the Empire. After the Liberation of Noriko Mochizuki, the JSDF bombed the Imperial Senate as a warning of the consequences if the Empire failed to release all Japanese citizens held in slavery. Meanwhile, another former slave of Zorzal's, Tyuule plotted to drive the Japanese to use their far superior firepower to destroy the Empire, leading to a botched Assassination Attempt on Noriko Mochizuki in an attempt to frame the Empire. She later was involved in the poisoning of Molt Sol Augustus, allowing the violently anti-Japanese Zorzal El Caesar to seize power, hoping he would drive the Empire to war.
Among the dark elves, there is a tradition of voluntarily placing oneself into the service of another person as a form of payment of a debt. This was done by Yao Ro Dushi after Itami saved her live during the attack on the Flame Dragon.