I loved Sgt. Frog.
I just read it and it's about a small frog alien, Keroro, whom will eventually meet his fellow frog aliens through the series, one which will be evil, who are determined to take over the earth. I laughed while reading this one. BTW, the humans are called
pokopenians.
Of course, the human boy, Fuyuki, and Keroro become close friends. You also have another girl, Momoka, who has a crush on Fuyuki; she also has a split personality, just like the frog she finds, Tamama, a subordinate and loyal follower of Sgt. Frog. They produce quite a bit of the comedy, both having split personalities, going from psychotic to sweet with a drop of a hat.
The mom - who is a crazed and works for a manga company - takes in the frog and soon they all become like his family. The mom was so funny. She decides to keep the frog for research for her manga, and even gives him his own room, which happens to have a young girl ghost - which shows up at unexpected times.
The sister of Fuyuki, Natsumi, hates the frog and never warms up to him the way the mom and brother do. And, everything bad seems to happen to her.
If you like comedy titles this one is a good read. It's rated Teen or 13+ and Tokyopop did a great job on it.