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 hanging 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.
Tenser-kun is a self-made multi-billionaire. Both arrogant and stylish, he likes to lead women on without making a serious commitment to them. Polygamous by nature, Tenser-kun recently had a falling out with Nefer-tan over the nature of their relationship.
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.
Adhara-tan is the Russian half-sister of Dawn-kun, often found in the science labs or computer labs after hours working on something or other. Member of way too many clubs and obsessed with bettering herself all the time.
Dawn-kun is angry at the world and the half-brother of Adhara-tan. The pair hate each other. He doesn't hide his affiliation with unsavory types like the Magnate gang at all, and is about as subtle as a brick to the face. Not very well liked. Despite this he is without question the best biology student in Theta High.
Tiger-tan is often something of an urban legend. Considered to be a ghost that haunts the upper corridors of Theta High, Tiger-tan is rumoured to be a graduate from the previous year who never left. In actuality, she is a snooty woman who runs Theta High's gossip mill, and is rarely seen outside.
Ham-tan also known as "Ham-Ham" to the other members of Zodiac Outer Classroom's Union is the treasurer for said organization. She is best known for her love of money and the luxuries which it buys. Fond of wearing the latest fashions, she is often selling latest technological gadgets of her own design to Ophen-tan and Heaven-tan. She is also rumored to be a ninja due to her uncanny knack of appearing out of nowhere and disappearing at a moment's notice when trouble starts.
Valera-tan is a highly likable girl that underwent puberty way too early and has a great deal to show for it. Tries to be friendly with everyone save for the Magnate gang but sometimes tries too hard and thus is often dragged along by some of the rowdier -tans on hijinks.
Red-tan is a delinquent who uses boku and fists. Green-tan is the student president. Blue-tan is the lead singer for Girl Sphere. Yellow-tan is Green-tan's emotionless assistant. Violet-tan loves super-hot natto tofu.