I Was Reincarnated in Another World and Auctioned off as a Slave
Shu is a Japanese office worker who isekaied right into a sex slave auction where he was bought by the prince Jadeite who never had a slave before. Luckily, sex slaves are really well treated, and Jadeite is really awesome, and the two work together to find out where Shu is originally from in this world and even try to find his lost brother. But not only is Shu being targeted by dangerous people for being a dark-haried "rare blood," he's also starting to lose his memories of Japan as he remembers more of this world, sparking him and Jadeite to worry they'll be separated when he remembers it all. Surprisingly nuanced and interesting despite all the comedy going on in the harem. Trigger warnings: sex slavery, kidnapping, attempted rape, mention of cannibalism and genocide.
Harem De Hitori