Hideki is an 18 year-old farm boy, who applied for University in Tokyo, but failed to pass the entrance exam. He goes to Tokyo anyway to study at the Yobikou (cram school) and has to get a job working part-time at a pub called Yorokonde (which loosely means 'gladly' or 'would love to'). Having lived on a farm all his life Hideki has a bad habit of talking/shouting to himself, which causes him much embarrassment when he hits the big city. Hideki isn't the smartest guy around either, but he does have a good heart, he genuinely worries and cares about everyone he meets and he also works hard at his job, though he does tend to fall asleep in cram school he works hard there too! Hideki has hormones like every other guy his age and often reads porn mags and lusts over the pretty ladies he meets, but would be absolutely mortified if they knew it and he constantly stresses over the fact that people might see him as some kind of pervert, especially since his innocent persocom Chii gets him into a lot of sticky situations.
Chii was discovered on a pile of rubbish by Hideki and was taken back to his apartment. She doesn't remember anything from before Hideki found her and so must learn everything from him. Chii is just plain cute and some of the misunderstandings she has are hilarious. One of the mysteries behind Chii is that she is a custom-made persocom (though it's not clear in the beginning who made her) and she appears to be able to move at the beginning without an OS (Operating System). She also packs a wicked CPU (or processing power), which allows her to cross servers and simultaneously connect to all the other persocom like a super-computer. This leads to the suspicion that she may be a 'Chobits' - a legendary persocom said to be able to think and feel independently just like a human.
Shinbo is Hideki's next-door neighbour at the apartment block he moves into. Shinbo is the same age as Hideki and they both end up going to the cram school together. Shinbo is fairly knowledgeable about persocoms and owns a small mobile form called Sumomo (which I gather he built himself though I could be wrong). It's Shinbo that Hideki generally runs to when he's in a mess, as Shinbo seems to have quite a deal more common sense. Shinbo is the one who puts Hideki into contact with Minoru-kun as well. You don't find out much about Shinbo from a personal angle until later in the series. He falls in love with their teacher Shimizu-sensei shortly after starting at the cram school and tries to convince her to leave her husband and remarry herself to him when he discovers how desperately unhappy she is. Shinbo's the kind of for-bearing friend we all need around to keep us on the straight and narrow.
Sumomo is the genki energetic persocom of Shinbo. Sumomo sings lots of songs for Hideki that Shinbo has made up and dances around when she's on standby. She sometimes becomes scared of Hideki when he's in his awed or psycho mode (not that I blame her). Sumomo generally lends a hand to our main characters, directing Chii to the shops or Hideki to the Peep-show house (wouldn't you love to hear more about that one! Dirty buggers.), taking phone calls to and from Shinbo and Minoru, as well as helping clean house and waking them up in the morning with lovely strenuous exercises. Sumomo's not what you'd call a complicated character, so you can't exactly get excited over her, but she does have her moments, so you really can't do without her when it comes to lightening the mood!
Minoru-kun is the rich boy-genius Shinbo directs Hideki to when he first finds Chii. Minoru is a well-known authority on custom-made persocoms and goes by the handle name of 'M' when on the net. He tells Hideki about the Chobits legend and posts a notice on the Persocom BBS (computer notice board) in order to help Hideki find out more information about her. Minoru has four custom-made Maid Persocoms and a custom-made persocom named Yuzuki, which he modelled in the likeness of his sister, who died when he was younger. It soon becomes clear that Minoru-kun cares for Yuzuki much more than he would a sister, though he seems reluctant to admit it and even goes so far as to warn Hideki in the beginning not to fall in love with Chii, no matter how cute she may seem. Minoru-kun puts a lot of work into helping Hideki as he constantly searches the net and rarely seems overjoyed therefore when Hideki calls him up to report the latest catastrophe.
Yuzuki is a high-performance persocom put together by Kokubunji Minoru. She was built to resemble Minoru's Onee-san (older sister) who died and has data installed that also helps her act more like Minoru's sister too. Yuzuki is at the centre of the Kokubunji residences system and therefore has control/authority over the other Maid Persocoms. She does the house-keeping and accounts and helps Minoru-sama with his research; enjoying a close relationship with him due to the resemblance she has to his sister. Yuzuki often feels that she isn't doing enough to help Minoru-kun and wishes she could be more useful to him. Even though she's not a 'Chobits' it does seem that Yuzuki genuinely cares for Minoru-sama.
Hibiya-san is Hideki's new landlady (Kanrinin-san). She runs the apartment block called Kabu Jougasaki and often lets Hideki off lightly when it comes to paying the rent. Hibiya-san gives Hideki clothes for Chii to wear and sometimes cooks him things to eat like Kinpira. She also teaches Chii how to cook and fixes Sumomo when Hideki electrocutes her.