All I know is Roxas would be the 13th forgotten sign, the one that wasn't included in the zodiac because 360 degrees doesn't divide evenly into 13. I just know it's supposed to be somewhere next to Scorpio.
I've never taken the time to compare the Organization members personality's to the zodiac signs and their respective qualities, but it's crossed my mind as an astrologer. But for some reason, I think Marluxia might be Capricorn. His scythe gives him a relation to Saturn, the ruling planet of Capricorn. However, Saturn also rules over Aquarius in joint with Uranus so I'm not sure.
Okay, just out of boredom, I whipped out my astrology text books and gave it some thought.It was a bit difficult considering all of the Organization members except Roxas lack in true emotion, so comparing them to metaphors for certain personalities that are more likely to exhibit certain emotions was a bit of a hurdle. Here's what I came up with.
I. Xemnas - Capricorn
Prudent, ambitious, disciplined, patient, careful, reserved, pessimistic, fatalistic, miserly, and grudging.
II. Xigbar - Aries
Adventurous, energetic, enthusiastic, confident, dynamic, quick-witted, impulsive, foolhardy, daredevil.
III. Xaldin - Scorpio
Determined, forceful, secretive, obstinate.
IV. Vexen - Taurus
Persistent, determined, placid, security-loving, jealous, possessive, resentful, inflexible, self-indulgent, greedy.
V. Lexaeus - Pisces
Compassionate, selfless, unworldly, intuitive, idealistic, vague.
VI. Zexion - Virgo
Modest, meticulous, reliable, diligent, intelligent, analytical, overcritical, harsh.
VII. Saix - Aquarius
Honest, loyal, independent, intellectual, intractable, contrary, unemotional, detached.
VIII. Axel - Leo
Generous, warmhearted, enthusiastic, faithful, pompous, patronizing, intolerant.
IX. Demyx - Sagittarius
Optimistic, freedom-loving, jovial, good-humored, straightforward, careless, irresponsible, superficial, tactless.
X. Luxord - Libra
Diplomatic, urbane, charming, easygoing, sociable, indecisive, changeable, self-indulgent.
XI. Marluxia - Gemini
Adaptable, versatile, communicative, witty, intellectual, eloquent, youthful, superficial, inconsistent, cunning, inquisitive.
XII. Larxene - Cancer
Intuitive, shrewd, changeable, moody, touchy.
XIII. Roxas - Ophiuchus
Unfortunately, due to Ophiuchus' removal from the zodiac, Ophiuchus is simply shared between Scorpio and Sagittarius. Therefore, there is no personality exclusive to Ophiuchus.