Sadamitsu used to be a typical delinquent, doling out justice as part of the Corpse Gang. That is, until he finds himself in the middle of a battle between a robot from space and an alien, inadvertently crippling the robot in the process! The robot is one of the good guys from another world, out to apprehend the 20 million galactic criminals that are on their way to Earth - and now that only its head remains intact, it's up to Sadamitsu to take on the challenge. While donning the head of the robot - which Sadamitsu names Junk - the boy transforms into a powerful fighter capable of taking down the invaders. With Junk's guidance, Sadamitsu helps fulfill his new role - but with the arrival of a powerful enforcer known as the Vulture, his task won't be easy...
Birdy Cephon used to be a butt-kicking intergalactic Federation investigator who took down bad guys where they stand; Tsutomu Senkawa used to be a normal student with shy tendencies and a penchant for exploring abandoned houses. However, when Birdy accidentally killed Tsutomu while trying to apprehend a famous criminal on Earth, Geega, both of their lives changed forever! Now, while Tsutomu's body is reconstructed in a faraway place, the two must share the same body – Tsutomu controlling it by day, and Birdy taking over when she spots a criminal in need of tough justice. All Tsutomu wants is to live a normal life; can he manage to cope with Birdy's enormous eating habits, intense muscle use, and easily irritable attitude?
In addition to the minor similarity in plot (otherworldly police person ready to apprehend space criminals? check!), both titles are just remarkably similar in tone. Each is a sci fi/action story but with a good dose of humor and plain old fun. It could be a gut feeling, but if you liked one try out the other.
In the year 2070, a giant object appears over the skies of the small town Tenmo, scrambling the electrical grid and confirming the existence of extra terrestrial life. To young Hajime Morata, this news is exciting and new - but to the townspeople, school council members and even the new exchange student Muryou Subaru, nothing going on in the skies is out of the ordinary. For it seems that Earth houses both intergalactic ambassadors and planet defenders alike - at least one of whom can use the power of the giant Shingu to battle incoming invaders - and Hajime is somehow connected. In addition to helping plan school festivals and living his day to day life, Hajime slowly begins to discover the secret behind his town and himself.
This could be a stretch, but I think there's enough of a similar feel to warrant a recommendation. Shingu and Sadamitsu are two sci fi tales that also seem surprisingly 'normal'. The main characters often embark on daily, non sci fi tasks and the action is coupled with a heavy dose of fun. Shingu is a lot more slice of life than Sadamitsu, and this may just be a gut feeling, but try it on for size.