If you're looking for manga similar to To Love Ru: Darkness, you might like these titles.
20 years ago, Weve Aliens arrived on Earth to search for marriage partners. Weve are perverted and love interbreeding. With their advanced technology, they have solved all the Earth's problems to do with the environment, food, and resources. Gen is a student at the prestigious Human-Weve school Shinryou Academy. Due to his late grandfather's words, he loathes Weve and believes that pure humans will eventually die out if something isn't done about Weves soon. However, Gen meets the Weve girl Mirika, who falls in love with him.
Two aliens are sent to Earth to scout for resources: that's how their adventures in Tokyo begins.
Emily, a transfer student, arrives at Kenji's high school. Kenji falls in love at first sight with the new student, but Emily is in fact an alien from the planet Chiara, an agent with the role of investigating the Earth in the form of a human mimic. As their civilisation developed, they began to degenerate as a species and eventually lost even the instinct to mate, so they came to Earth to learn about love and sex in order to ensure the survival of their species.
This is an in-your-face romcom with a mix of nudity and naughtiness. Caring for people is deeply ingrained in our protagonist, whose father is a butler and mother is a maid. Before him appears a rich young lady who is helplessly unable to do simple, everyday tasks. This is the start of a restless life with a young lady who cannot even change her own clothes!
Yuto follows Mikoto, a childhood friend and high school girl who is one year older, to enter the same high school. When she goes home with Mikoto for the first time in a year, it turns out that Mikoto only sees Yuto as her younger brother. Yuto is a little dizzy about the difficult road ahead, but an alien "onee-san" in a very erotic outfit appears in front of the two...!?
Shibata Youichi is an ordinary high schooler, but one day he runs into a mysterious heroine! The cute girl knows his name, and (as in a stereotypical anime) she's an alien who needs him to help save her people! Soon enough, Youichi finds out that this situation was too good to be true... The girl, named Kyara, does represent aliens, but her people are actually interested in Japanese manga/anime/game culture. They want to use his home as a base and his brain as a measuring tool of the level of appeal of their heroines!