Mutsumi Houjou

Saburo, also known as 623 (Mutsumi) or Mutsumi Houjou in the manga is a main character in the Keroro Gunso series.

Mutsumi is a high school student in the manga, while Saburo is a middle school student in the anime. He has an I.Q. of 350 and is Kururu's partner, due to having similar wavelengths. He is the least-active main character of the series, but plays a fairly significant role as Kururu's partner/friend and Natsumi's crush. In the anime, Saburo's identity as Kururu's collaborator is at first a secret (though it's quite obvious), whereas in the manga it comes almost immediately to light.

He skips class a lot and is rumored at school to be a member of the "Men in Black", though that doesn't seem to affect Natsumi's high opinion of him. Saburo is excellent in sports, academics, poetry, art, and philosophy, which is why he has a hard time fitting in with his classmates. There are even rumors about him being an alien. He hosts a late-night radio program which Natsumi compulsively listens to.