Sphere-tans

The specific location of Sphere-tan's city is unknown. Various clues including the two moons, the local gas giant and various Earth landmarks (Tokyo Tower, Arc de Triomph, the Forbidden City, the Twin Towers II and St Basil's Cathedral) not to mention that in several episodes the entire setting appears to be inside an O'neil cylinder point to a great deal of discontinuity happening.

Earth-kun is the faceless protagonist and unreliable narrator.
Daedie-tan is a scowling girl with long red hair and eyepatch, glaring with a single blue eye. She can often be found poking up from behind furniture or out of gratings, stealing cookies and other valuables. Give her sugar cookies and she'll spare you.
Londen-tan is a yamato nadesico, the mother figure and president of the Zodiac Outer Classroom's Union.
Minkowski-tan lives in the ceiling space, occasionally removing a panel and dropping down to dispense religious literature. You need to max your zero-g skill to date her.
Nefer-tan is a young scarlet haired woman with a tragic past. Formerly best friends with Tenser-kun, they had a falling out recently during the power outtage known as 'The Breakdown', whereupon they parted ways. Nefer-tan is attended by a butler named 'Mister Pyramid', whom dresses in a pair of white gloves and an immaculate suit- but whose face is never shown on screen.
Silesea-tan is quiet and stoic, with a faint german accent. At lunchtimes she eschews society to hide in various out-of-the-way places, but is always dragged by chance into the day's hijinks anyway.