Characters in a nutshell:
Unless noted, all characters are 17 years old!
Ataru Moroboshi: Unlucky student, who probably deserves it. "Moroboshi" means "falling star"; additionally, "shi" is both the word for "death" and for "four", and consequently four is an unlucky number in Japan (and one often associated with Ataru). He lusts after every girl, except of course Lum; much of his free time is spent either lazing around, hunting after girls, or devising new ways to ditch Lum and go hunting after girls. His grades are low-average, he doesn't have many friends, and he makes a habit of torturing Ten, Lum's young cousin. However, Ataru seems to almost intentionally hide a generally good nature: when motivated, he will offer unselfish help to people who need it, and he's not actually as dumb as he looks. He does, however, have a genuinely irresistable need to come on to every female in sight, as long as she's good-looking, so his offers of help to those in aid usually come with a subsequent molestation attempt. ;) The only character who has ever seemed to see anything worthwhile in Ataru has been Lum; naturally, he avoids and ignores her, although deep down he loves her more than any other girl. He knows this, and shows it in spontaneous actions, but usually tries to hide it whenever he can.

Lum: Alien princess. Lum started out as much more "demonic" looking, fitting the profile of a female sort of Oni in Japanese legend: the ogres in folklore can sometimes take the form of beautiful women with very hot tempers. *G* In early stories, Lum's entire design was more exotic, and her temperament more demonic; even the way she showed affection for Ataru (to whom she immediately gave the pet name "Darling") was painful, since it usually involved tight cuddling and electric shocks. Later, Lum used the shocks only as a form of punishment, but they still were used pretty frequently. Lum's attitude changed markedly over time, going from jealous and somewhat wild to good-natured and cheerful; she remains devoted to Ataru, but still has something of a jealous streak. She is consummately optimistic, believing that soon Ataru will want to settle down and share her vision of a calm, loving (and monogamous) future. Her treatment of Ataru is usually overwhelmingly loving, as she wants to feed him, nurture him, and spoil him rotten; on the other hand, it can't be forgotten that, in addition to this behaviour being somewhat clingy, Lum also can easily change face and in an instant punish him for whatever transgression she thinks he's guilty of. She's also entirely unaware of the danger her affection for him poses to Ataru himself, thanks to the other male cast members who are in love with her, and would like Ataru out of the way for good.

Shinobu Miyaki: Ataru's girlfriend at the start of the series. Her name ("Miyaki") means "to like"; not to mention, there's that unlucky "shi" plaguing Ataru again! ;) A sweet girl who puts up with a lot, until she reaches her breaking point. She's rather quickly "dumped" - or maybe she does the dumping - though Ataru quite often makes attempts to win her back. However, once Shutaro Mendo enters the series, Shinobu has moved on to better pastures than what Ataru can offer. Shinobu is, as mentioned, a tolerant and sweet-natured girl - but her temper rivals Lum's, and at times she has an even shorter fuse. When she's angry, her strength multiplies, and she is known to throw desks and other large objects, usually at Ataru. Shinobu is smart and a romantic at heart, but remains mostly unfulfilled in her life. She and Lum are rivals even after she's moved her interests on to Mendo, mainly because of Ataru's continuing interest in her and Mendo's continuing interest in Lum. Towards the end of the series she finds an admirer in Inaba, a dream-maker who she befriends on the street, and they are subsequently seen dating at the end of the series.

Shutaro Mendo: The rich, handsome, suave scion of the Mendo family, one of Japan's most powerful families. His name means "trouble". Mendo swoops down a short ways into the series and captures the heart of most of the female cast, with the exception of Lum - who is, of course, the only girl he is interested in. Despite his good grades, luck with women, and deep bank account, Mendo is actually pretty much the same as Ataru underneath: fickle, interested more in girls than in relationships, and a bit self-serving. If anything, Ataru might be more motivated towards the "greater good" than Mendo is. *G* Mendo, naturally, hates Ataru and considers him to be his only rival - and worse, Ataru is so distasteful and common that he shouldn't even be an issue. Ataru, on the other hand, is irritated by Mendo but doesn't seem to consider him much at all when they're not around one another. Despite all of this, the two boys spend most of their time in the same social circles, always work together in class projects, and go to the same parties. ;) "Best enemies", I suppose. When it comes to Mendo, Lum shows no real interest; she treats him as a friend (which she does for everyone), unless he makes some threat towards Ataru - then Mendo is fair game.

Sakurambo, aka "Cherry": An old, strange monk who follows Ataru around proclaiming him to be the unluckiest boy in the world. His name means, well, it's a pun actually: "Sakurambo" actually means "deranged monk". However, "Sakura" means cherry blossom, hence his self-styled nickname. Generally, Cherry is more an annoyance than anything else, though he does cause Ataru trouble when he pops up - at times only through his huge appetite and tendency to freeload.

Sakura Sakurambo: Cherry's niece - older than Ataru, probably in her 20s. As mentioned above, her name means "cherry blossom". Sakura is extremely beautiful, and has strong spiritual powers like her uncle. She is a miko, and also works days as the school nurse at Ataru's high school.... In part to stay near Ataru, who attracts trouble like flies. She's engaged, much to the chagrin of every boy at the school, to a hapless wizard named Tsubame Ozuno (who doesn't appear much in the series). Sakura is also a trouble magnet, although she's much more useful with her powers than Cherry is.

Jariten, aka Ten-chan: Lum's young cousin. Evidently, if appearances are to be followed, he is very young, possibly about four years old or younger. He usually speaks and acts precociously, though. Ten is a little boy with a bowl haircut and a single horn on the crown of his head; he wears tiger-striped shorts (which might be diapers), and flies unsteadily. Like Ataru, he's girl-crazy, and loves being cuddled by women; he gets away with it, though, because of his youthful appearance. Ataru torments him constantly, and Ten defends himself in the way that comes most naturally: by breathing fire. Ten and Ataru are constantly at odds, since in addition to Ataru's bullying and jealousy over Ten's cuddliness, Ten has contempt for Ataru's stupidity and really dislikes the way he treats Lum. Deep down, however, they seem to have more of a brotherly relationship than either one likes to let on.

Ryuunosuke: A girl who was raised as a boy. She therefore looks and acts like a boy, practices martial arts, and is unable to relate to boys in order to date them, even though her deep-down wish is to be a normal girl. Even knowing that she's female, most of the girls at Tomobiki high school have something of a crush on her.

Lum's Stormtroopers (anime only):
Megane: the leader. His name means "glasses". He's the leader of the high school group dedicated to pursuing, adoring, and protecting Lum. The most articulate of the four, he is also closest to Ataru, and was given the most character development. Still, however, when it comes to winning Lum, he never has a chance. *G*
Perm/Paama: Second in command, probably. Tiny bit of character development, in terms of being something of a goof. His name means - well - "perm", because he has curly hair.
Chibi: Little, high-voiced guy. His name means "runt". Usually picked on by the other three.
Kakugari: Big burly guy. His name means "big burly guy". Usually tormenting, or alternatively carrying around, Chibi.

Lum's friends:
Benten: Like the Oni, Benten is derived from Japanese folklore, in this case the Seven Lucky Gods. This Benten is another alien princess (and she drives a flying motorcycle and wears chains), and she and Lum are old friends. Benten likes big guns, and is a bit boyish, but is hot; Ataru is, naturally, smitten. Benten is completely uninterested.

Oyuki: Princess of Venus. Her name means "snow". Beautiful, and cold, but she warms up to Ataru more than any of Lum's other friends. She's a shrewd business woman, smart and calculating, but also soft-spoken and - at first glance - more traditionally Japanese than Lum's other friends. That's just a cover. *G*

Ran-chan: A cute, pink-haired girl, possibly from Lum's planet. Her name means "chaos". No one except Lum seems to know that she's an alien, at least not for a while. Ran is a bit on the MPD side, it seems; she was Lum's best friend growing up, and through her we learn that Lum was a holy terror as a child. Poor Ran got the bulk of the blame for most of it; not only that, but Ran's mother had a nasty temper, and so when Ran got the blame she really got the blame. Ran is alternately determined to make Lum miserable - usually by stealing "Darling" away from her - and sweetly interested in making up with Lum. Her good moods rarely last, however. She's actually in love with Rei, and is happiest when she can feed him (which can go on for hours). She has the power to suck a person's youth out through their mouth, so she usually tries to attack Ataru by kissing him. Most everyone who knows Ran these days considers her an airhead, and a loony one at that - except Ataru, of course.

Rei: Lum's former fiance from her home planet. His name means "zero". An incredibly good-looking young man who dresses in a full-length tiger-striped bodysuit. Every woman who meets him is smitten, even in the face of his monosyllabic vocabulary, bottomless appetite, and horrendous manners - at least until he turns into a giant tiger cow, which is usually what happens when he gets emotional, excited or upset. He still loves Lum, but she is fed up with him, and wants nothing to do with him. Then comes the tiger cow.

These are not all the characters, by any means; UY has a huge cast. But these are the main ones to know, introduced within the first few episodes and/or chapters of the manga.