Taurus:
Taurus is ruled by the bull, and like the bull that represents them, Taurus people tend to be stubborn, loyal, etc.  An earth sign, Taurians enjoy all of the good physical pleasures like good food, good drink, and the opposite sex.  Taurians tend to be good people, and, though they tend to be fairly slow in making up their minds, they do not tend to be stupid…
Character:  Gourry
…unless, of course, they are Gourry.  Well, Gourry is certainly slow and rather bullish.  He is quite stubborn (note how Lina still doesn’t have the sword of light) and certainly enjoys eating and looking for cute babes who are (hopefully) a little bustier than poor Lina.  Like any good Taurus, Gourry is robust and healthy and is certainly loyal.  (How else would he stick around the abusive Lina for so long?)

Gemini:
Gemini are, as their name might suggest, dual natured.  Their tempers are "Dr. Jekyll and Mr. Hyde"ish and their interests are varied.  They love talking, activity, and this sort of thing and are constantly doing something.  Gemini are agile witted and the tricksters of the zodiac.  They tend to be a bit nervous and jittery.
Character: Xellos
Anyone called the trickster priest is, by their very nature, a Gemini.  How could it be otherwise?  The good/bad nature of the character also fits the duality of the sign.  Besides, how else could someone be cast who’s favorite line seems to be "It’s a secret".

Cancer:
Cancers are the clingy signs of the zodiac.  They’re emotional, nostalgic, loving, spineless, clingy, and sympathetic. They’re pretty much the sign that you know that is basically a nice sign, but has a tendency to get over indulgent and over emotional.  Poor Cancers.  Cancers love home, family, friends, and all of that prosaic stuff.
Character:  Sylphiel
This does not even need an explanation.  From the saccharine "Gourry-sama" to being one of the few characters in Slayers that doesn’t seem to run roughshod over other character’s feelings, Sylphiel is a Cancer.  Sylphiel also is sweet, cares about her family, and seems upset when most of her city is destroyed.  In accordance with the sign, Sylphiel is a great healer and a sensitive psychic.

Leo:
Leo, the lion, is a king of the zodiac.  Of course, this means that besides an inherent regalness and all that, Leos also tend to be egotistical and power-hungry.  Leos tend to be athletic and shine as the star in anything they do.  However, they can also be blunt, abrasive, and do not withhold their opinions.
Character:  Zelgadis
Zelgadis certainly never holds back an opinion and his desire for power made it possible for Rezo to change him into a chimera.  He is, however, a kingly character. There is a dignity about Zel that there isn’t about many of the characters.  Although Leos are not known for being shy, all of the Leo males I’ve known can be endlessly tortured by questions about their love lives.  ; )

Virgo:
Virgos are obsessive, analytical, perfectionist people.  They like to have things a certain way, and tend to get it. (Control Freaks!)  Virgos love science, education, and all sorts of neat mental things.  Virgos tend to be quiet, painstaking, and modest, but can also be fussy.  Virgos tend to get along with people like them (usually other virgos or sometimes capricorns or tauruses) and will often get incredibly attached to an intellectual equal or superior and tend to prefer platonic or near platonic relationships.  Virgos also have a fascination with health.
Character:  Eris
Eris with her love of kopiis, well worked, detailed plans, and absolute fury whenever anything goes wrong could be a happy virgo.  She is certainly obsessive, and, although her little black dress is not particularly modest, her earlier dress is one of the more modest feminine outfits in anime.  Eris is certainly an intellectual character who refuses even to fight unless she must (it’s so much easier to send out kopii!) and spends a good portion of her time studying, working with her kopii machines, and trying to find ways to get ahold of new things to study.  She also seems to have this obsession with control, as might be seen in her little red gems that all of the poor kopiis are implanted with.

Libra:
Libras are artistic people who love beauty and harmony.  They need love to be happy. They are aristocratic and refined.  They can be egotistical and can be very bizarre when it comes to sex.
Character:  Filia
Filia’s love of beauty and harmony are very apparent, especially when she goes into her ceramics stage.  She needs to be happy and hopes that somewhere is the right sort of person for her.  She’s certainly aristocratic and refined, from her delicate tea drinking to her demands that everyone behave themselves.  However, strangely, her fan’s choice in partners for her tends to fall on two of the strangest male characters, Xellos and Valgarv…hmmm…what could be going on here?

Scorpio
Scorpios are extremists.  They are determined, driven, and self-willed, even to the point of rebellion.   Scoropios tend to be inflexible and difficult to convince to do anything other than whatever it is that the Scorpio wants to do.  Scorpios like to work and feel busy and tend to feel that they must get the confidence of others.  Scorpios have good minds and are usually deeply philosophical or profound.
Character: Amelia
Amelia is an extremist. She’s even called a "justice-freak" by some of her adoring fans.  She’s driven, determined, and self-willed to a fault and can be quite inflexible.  Amelia also is quite intelligent, when she does not allow her passions to sway her.  She also has a strong desire to feel needed and a very strong work ethic.

Sagittarius
Sagittariuses are athletic, adventurous, outgoing, and tend to be popular.    A negative Sagittarius will be boastful and bragful, continually working on "get rich quick" schemes and the such.  A positive one, though, will be one of those marvelous people who always seems to be having a good time.
Character: Nagha
Although virtually all of the characters in Slayers are athletic and adventurous, Nagha is not only that, but also demonstrates some of the not quite so good Sagittarius qualities.  Nagha, despite her thrill for adventure, is always after treasure and is always boasting and laughing.  Nagha is very athletic, having a body that actually can support a tiny leather bikini, and her laugh would certainly indicate, if nothing else, a healthy set of lungs.

Capricorn
Capricorns are the businessmen of the zodiac.  But they are also the scientists and the priests and the practically anything that involves power, intelligence, and ambition.  Capricorn is perhaps the only sign, other than Leo, that wants power for power’s sake.  Capricorns love to work, and are never really happy unless they have something to do.  A good Capricorn will be an outstanding person in their community with a healthy family, a good job (which they work hard at) and all of that. However, the power-hungry side of Capricorns can be very negative.  Capricorn is the sign of the devil, and can demonstrate itself as such. Capricorns are the best villains, and often carefully work around rules in order to get what they want, at any cost.  However, all Capricorns are very serious and hardworking, no matter what, exactly, they are working towards, although they are often known for a wry sense of humor.
Character: Rezo
OK, I admit it, if I didn’t need a Leo, this would probably be an ideal sign for Zel, too.  But…it’s an even better sign for Rezo, don’t you think?  Capricorn really is the sign of a villain, and of a good villain at that.  Rezo certainly thirsts for power, and works very hard to get it.  He’s ruthless and intelligent and all of that.  Not to mention, he’s a patriarchal figure and just fills it so nicely. ; )