Characters in a nutshell:
Yusaku Godai: Approximately, what, 19 at the start of the series? The main protagonist, most of the series is through his eyes. The "Go" in his name means "five", and refers to the number of the apartment he lives in, as is the case with most of the Ikkoku tenants. Godai is an earnest young man who truly wishes to do well, but has a very hard time applying himself to reach the goals he most desperately wants to attain. Added to that is some really dreadful luck on the occasions when he does apply himself, and you can understand why most of the people who know him consider him pretty unreliable. Godai is pretty unaggressive, and has a lot of trouble saying "no". Well.... that's not entirely true; he says no, but he has trouble acting on it. He also has no real goal or career path to start with, and it's not until quite a ways down into the series that he discovers he has a definite way with children, especially younger ones. For all his bad qualities, he has at least one equally strong good one, which (combined with his passiveness) is probably why he attracts women enough for them to boss him around some, but not quite enough to actually benefit him in any way. In any case, his love for Kyoko is never fully explained, except that she is beautiful, slightly older than he is, and a relatively tragic figure - also a more mature woman than any he's known, and one who shows him some kindness early on. Whatever the reason he falls for her, he's pretty gone on her from the start, and it only gets more deeply ingrained from there. His long-term goal is to prove that he's not really the unreliable slacker he was when Kyoko came to live at Ikkoku, and to get her to see him as a man who can provide for a family - a man she'd like to raise her family with.

Kyoko Otonashi: Two years older than Godai, approximately 21 at the start of the series. The name "Otonashi" means "nothing, zero" which is actually more a reference to her late husband, who is seen in flashbacks and photographs, but his face is never shown. However, in keeping with this meaning, Kyoko's apartment is the only one in Ikkoku without a number on its door. She takes a job at Ikkoku, which is owned by her father-in-law, as a way to move on with her life after the death of her husband. Understandably, it takes her a long time, even after resolving to be optimistic and happy, to let others into her heart. Her first impression of Godai is evidently one of a struggling student, possibly someone to be pitied and maybe even coddled a bit; over time, she comes to care for him more and more, and at some undetermined point in the series she does seem to come to return his feelings. However, Shun Mitaka is a far more aggressive romantic presence than Godai, and Kyoko, unused to being pursued so obviously by two determined suitors, can't decide between either of them. Kyoko is a sweet-natured, almost motherly woman who enjoys cooking for people and taking care of them. However, she has a powerful temper, and jumps to conclusions easily - in fact, sometimes she almost leaps. She's also incredibly possessive, and on many occasions gives Godai the cold shoulder just because she imagines he's interested in another woman - when she hasn't even given him an indication that she's interested in him. Interestingly, Kyoko's relationship with her parents is somewhat strained, especially with her mother, due mostly to her continued association with the Otonashi family after her husband's death. Kyoko's mother is smart but stubborn, and brings out the same stubbornness in her daughter. Ultimately, Kyoko is an odd mix of good and bad qualities, but the good must win out, since her admirers are so devoted. Kyoko has a shaggy, odd-looking dog (probably a terrier of some kind) named Soichiro, after her late husband; she adores him.

Shun Mitaka: The older, suave, lady-killer tennis coach who takes a shine to Kyoko. Mitaka seems throughout the series to make a good deal more progress with Kyoko than Godai, due in large part to his having a steady job and good income and thus being able to take her out on dates to fancy restaurants on a regular basis. He also has the self-confidence Godai lacks, and flaunts it. Mitaka holds Godai in contempt, and doesn't take him seriously as a rival at all; the idea that Kyoko could actually have feelings for Godai takes him by surprise. Despite his determination, however, he has two weak points: one is his reputation as a playboy, which does not change despite his declared devotion to Kyoko; the other is his unfortunate fear of dogs. He's phobic about them, which means he can't really successfully get close to Kyoko.

Kozue Nanao: A sweet girl, a couple of years younger than Godai, who worked with him over a summer and developed a crush on him. Kozue is the first girl to show a real interest in Godai, and the only one to act like a genuine girlfriend. She likes to go out with him, make him things, take him home for dinner with her parents. Kozue is definitely interested in a real relationship, and under other circumstances Godai would probably feel lucky to have her, but unfortunately, with things being the way they are with Kyoko... Oddly enough, Kozue remains entirely unaware of Godai's feelings for Kyoko, and never seems to realize his conflicted feelings towards her. She's happy just to be around him, most of the time.

Ibuki Yagami: A high school girl in the class Godai student teaches in while he's finishing school. She likes to think of herself as more mature than most other high school students, but after examining Godai - "Mr. G" as she calls him in the Viz manga translation - she decides he has a romantic soul, and develops a schoolgirl crush on him. Only Yagami is not your typical schoolgirl. She pursues Godai almost relentlessly, to the point of moving in with him at one point, and creates a rivalry between herself and Kyoko which Kyoko is eventually forced to take seriously. Yagami causes more trouble than she means to, but she is motivated by the passions of youth, and what she genuinely believes is true love; unfortunately all her plans backfire on her, and especially on Godai. Still, she never gives up, and her stories are some of the best in the series.

The Ikkoku tenants:
Mrs. Ichinose: The older woman in Apartment 1. ("Ichi" = one; there's some buzz that her name is also based around "nosy".) She's loud, boisterous, an incredibly busybody, rude, and likes to get drunk and dance on tables with fans. She lives with her husband, who is rarely seen and who is a very mild-mannered fellow, and her young son Kentaro - something of a brat, but embarrassed by his mother's behaviour and longing for a "normal" upbringing. Ichinose is a meddler, but she has genuine maternal instincts that sometimes come to the foreground... and other times, as she ignores her son and squanders money on booze, they don't.

Yotsuya: The tenant in apartment 4, next to Godai. ("yo" = 4.) Yotsuya is possibly the oddest of the tenants, with a definite perverted streak and a taste for voyeurism, but a rather inscrutable demeanor. He has created a hole in the wall separating his room from Godai's, and spends a lot of time mooching food off of Godai in exchange for not sharing information he's overheard or snooped his way into. Like the other tenants, he drinks quite a lot, more often than not in Godai's room - if Godai is trying to study, even better. During the day he usually wears a suit, and often leaves to go to "work", though it's never clear where he works or what he does (if anything). He gives an air of cultivated intelligence, but shows no signs of mercy or any other human decency. In fact, Yotsuya is the only name anyone knows him by - whether it's his first name or his last is a mystery as well.

Akemi Roppongi: The tenant in apartment 6, on the other side of Godai. The first kanji making up "Roppongi" is the same for "roku", which is the word for 6. Akemi is a young woman who works nights in a bar; she often accepts drinks from her customers and most nights comes home smashed. As a result she sleeps late into the day, usually wearing mostly see-through nighties, and will walk around in them well into the morning. Yotsuya has created another, smaller hole in the wall separating Akemi's room from Godai's, and will regularly invade Godai's room to peep on Akemi as she sleeps or dresses. Naturally, Akemi blames Godai. In general, due to Akemi's nature, it seems to be hard for most men to entirely disregard her; even though Godai is not fond of her personally, he entertains the occasional lustful thought towards her. Akemi, on the other hand, usually seems to consider Godai a joke and nothing more. Her love life is not a stable one, and as such she's quite the romantic cynic; she views most of the goings-on at Ikkoku with a sarcastic eye, but has an occasional flash of insight which she may share. Generally, with a certain amount of contempt.

These aren't all the characters, but they're most of the main ones appearing in earlier issues; some of the best ones come later, but I'll leave them as surprises for the time being. ;)