If you're looking for manga similar to Nekketsu Uchuujin, you might like these titles.
Watanabe, an alien student from 20,000 light-years away, comes to homestay with the Tokino family to gather material for his thesis on Earthling behaviour. He follows around the various family members asking mostly funny but sometimes profound questions about their work, their loves, their polluted air... When he's not being sent on errands or punched in the face, at least.
Dr Kim works at the Korean equivalent of NASA. He has talent, brains and is unparalleled with his programming skills. One day, he hacks into the Mars Rover and catches the glimpse of an alien. Unfortunately, the device wasn't recording at the time and so he has no proof and clearly no one believes him. And as fate may have it, that alien has come to Earth believing she had been caught on video and wanting nothing more than delete that proof before her boss finds out. Misunderstandings ensures as the two characters meet and are forced to work together as a new group of aliens start stirring trouble on Earth.
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!
This is a story about aliens. Yes, aliens. Choi Sera houses displaced aliens in her home, where they all pretend to be UFO fanatics in order to explain their eccentricities. Sera’s job is to teach these aliens how to live in human society—not that difficult on the surface, as they ostensibly look like humans (except for the three fingers on each hand thing). They look normal, and they can communicate with earthlings via a translator that looks like a hearing aid, but can they act normal? The answer to that is a resounding no. Watch as Sera tries to school the emotionless twins, Seunghyun and Seungmin (really clones) on dating (with disasterous results), tries to keep Hyunwoo out of trouble, and keep them all from being taken away by the UN alien hunter who’s on to them. And what happens when a translator breaks?!
Tsutsui Yukitaka is a freshman entering high school who hopes to become a good baseball player. However, when he walks into his new apartment, there is a man in the room who claims that he is an alien who lost his memory. He is in fact the first prince of Planet Dogra, known to have the best brain but the worst nature.
On his way home from school, the shy young boy Tecchin finds the body of a young girl lying in the street. He alerts the police, only to come back to find she has been replaced with a life-sized doll. Tecchin takes the doll home and repairs it. She comes to life and tells him that they are both Grand Dolls, alien beings bent on world domination.