Arumi Asahina
Sasshi’s best friend and classmate, having practically grown up together with him in the Abenobashi Shopping Arcade. A sensible and pragmatic girl who is a foil to Sasshi. Arumi’s eccentric father and stubborn grandfather run a French restaurant in the Shopping Arcade known as the Grill Pelican. It appears, however, that the Asahina family will be closing up shop in the near future as part of redevelopment in the area and moving to Hokkaido, forcing Arumi to leave Sasshi behind. 
Satoshi Imamiya
Satoshi Imamiya, or "Sasshi" is a precocious, carefree, hyper 12-year-old boy who speaks with an Osaka accent and can be pretty dorky at times. He has a huge passion for collecting things, such as baseball cards, vending machine toys, and so forth. The loss of his treasured possessions when he moved to a new condo was hard on him, but it'll just motivate him to restore his collections to their former glory. He's also a big fan of RPGs, sci-fi, dinosaurs, guns, and dating simulation games. Sasshi's family used to run the Turtle Bath, which was the local bathhouse, but was forced to give it up and move out due to redevelopment plans for the Abenobashi shopping arcade area. Sasshi spends his lazy days hanging out with his best friend Arumi, having practically grown up together with her in the shopping arcade. 
Mune-mune
Sasshi and Arumi encounter this funky young lady in the various other dimensions of the Abenobashi Shopping Arcades. Extremely well endowed, much to Sasshi's delight, Mune-mune tends to be a very cheerful and bubbly person who makes the kids' lives quite interesting amidst their travels. Sometimes she's trying to kill the kids, others she's very nice to them. Mune-mune appears to be searching the various alternate dimensions for the mysterious man wandering Abenobashi in a Wizard-like robe, Eutus, who she seems to be quite fond of. 
Eutus
Eutus is a strange and mysterious man that wears a Wizard-like robe who the kids encounter wandering throughout the various dimensions of the Abenobashi Shopping Arcade. Eutus comes off as a wise know-it-all, and absolutely hates being called “Mister” due to the implied range in age, and would much rather be referred to as "Bro", or something similar. Eutus has a powerful connection to the Abenobashi Shopping Arcade which remains a secret, but in the meantime, he's doomed to wander the various dimensions of Abenobashi until the cause of his misfortune is rectified. 
Aki
The neighborhood cross-dresser who is obviously a man, but dresses in women’s clothing and wears a thick application of make-up, Ms. Aki has been good friends with the kids for a very long time now. While not much is known concerning Ms. Aki's background, this peculiar individual has been a resident of the Abenobashi Shopping Arcade for years, and seems quite knowledgeable about the history of the area and those who dwell there. Ms. Aki shows up in the different dimensions of Abenobashi, usually in the company of Sayaka or Mune-mune. Still, regardless of the dimension, Ms. Aki is not pleasing on the eyes.
Sources: Wikipedia | MyAnimeList | AnniDB