All quotes and info taken from Sydney Omarr's Astrological Guide For the New Millennium, Libra 2000 (I feel like I'm writing a research paper...Heeheheheheh...). My comments are in (red). Disclaimer: This is just for fun. I don't claim to be a professional.

All right, so no one really knows when Trowa was born, so I pinned him as Capricorn based on the descriptions.
Capricorn is the tenth sign of the zodiac. It is an earth sign, denoting people who are "solid, practical realists...They are interested in ideas that can be used to achieve concrete results." It is a cardinal sign, which means they like to be involved in projects and productive activities. Capricorn's polarity is reactive/negative/yin, giving them a "receptive nature that allows energy to develop and take shape".
Capricorn is in the tenth House of the Zodiac. The tenth house is also where Saturn resides. This placement "is the most 'visible' house in the chart, the one where the world sees you. It deals with your public image, your career (performing in the circus) (but not your routine "job"), (fighting in battles) and your reputation. (as a methodical guy who knows how to get the job done) Here is where you go public and take on responsibilities (as opposed to the fourth house, where you stay home). This will affect the career you choose and your 'public relations'. This house is also associated with your father figure or whoever else was the authority figure in your life." (In Trowa's case, this would be War)

Here's a bit about Capricorn from Astrocenter.com. It's quite clear why I think this is Trowa's sign: "Here is a stabilizing force, one of the hardest-working signs of the Zodiac. They have intense powers of self-concentration, but not in an egotistical sense. Members of this sign find a great deal more confidence what they do than in who they are. Earth hangs tough when it comes to stabilization, so achievement provides that stability and structure. Capricorn is one very high-achiever! Without balance, however, Capricorn can become too rigid -- as in the tall palm tree, falling to the ground, under heavy wind-gusts. To be able to move with the winds of change, Capricorns must bend and flex a little bit! That makes life much easier. Capricorn can also focus too much on achievement. Then they forget the little joys in life. When they finally relax and enjoy life, their most delightful secrets emerge. No one has a better sense of humor than the Capricorn. Oh, that Cap might let us bask in that warm smile! :-)" (and such a nice smile it is...)

Rising Sign - this is one of the most important factors in determining the uniqueness of your personality, outward appearance, etc. For Trowa:

"Aquarius Rising - One of a Kind. You come across as less concerned about what others think (Mr. Aloof) and could even be a bit eccentric. You're more at ease with groups of people (like a circus audience) than others of your sign, and you may be attracted to public life. (again) Your appearance may be unique -- either unconventional (must I mention the hair?) or unimportant to you (that clown outfit...) Those with the sun in a water sign (Cancer, (^_^) Scorpio, Pisces) may exercise your nuturing qualities with a large group, an extended family, or a day-care or community center." (watching over the circus kids ^_^;)

Now for the really fun part, sorting through the effects of the planets! Each planet, including the sun and moon, add pieces to your strengths and weaknesses to develop your full personality.

The sign the moon was in at your birth "reflects your instinctive emotional nature and the things that appeal to you subconsciously."

Moon in Capricorn: " Are you ever accused of being too cool and calculating? You have an earthy side, but you take prestige and position very seriously. A structured situation where you can advance methodically makes you feel wonderfully secure. You may be attracted to someone older or very much younger or from a different social world. (^_^) It may be difficult to look at the lighter side of emotional relationships, but the 'up' side of this moon in its detriment is that you tend to be very dutiful and responsible to those you care for. (ep 43 much?)

Mercury -- this planet "shows how you think and speak, and how logically your mind works."

Mercury in Virgo: "This is one of the best places for Mercury. It should give you the ability to think critically, pay attention to details, and analyze thoroughly. Your mind focuses on the practical side of things, making you well suited to teaching or editing.

"Venus is the planet of romantic love and artistry. It shows what you react to."

Venus in Cancer : "An atmosphere where you feel protected, coddled, and mothered (Catherine much?) is best for you. (best for him; not necessarily what he wants, heheheh) You'd love to be surrounded by children in a cozy, homelike situation. (he'd like a real home, at least) You are attracted to those who are tender and nuturing, (^_^) who make you feel secure and well provided for. You may be secretive about your emotional life (like burying all emotion) or attracted to clandestine relationships.

Mars "shows how you pursue your goals and whether you have the energy to burn or proceed in a slow, steady pace."

Mars in Cancer: "You rarely attack head on -- instead, you'll keep things to yourself, (such as emotions) make plans in secret, and always cover your actions. (such as when infiltrating the enemy...) This might be interpreted by some as manipulative, but you are only being self-protective. You get furious when anyone knows too much about you, (he does kinda scowl when he meets up with Quatre in ep 7...) but you like to know all about others."

Jupiter "brings good luck, gifts, success, and opportunities, and also embodies the functions of the higher mind, where you do complex, expansive thinking rather than the specifics."

Jupiter in Virgo: "You actually love those minute details others find boring. To you, they make all the difference between the perfect and the ordinary. You are the fine craftsman who spots every flaw. You expand your awareness by finding the most efficient methods and by being of service to others. (Infiltrating the enemy not once, but twice, la la laa...) Many will be drawn to medical or teaching fields. You'll also have luck in publishing, crafts, nutrition, and service professions. Watch out for a tendency to overwork.

Saturn "tells you what's wrong with you for your own good. Under a Saturn transit we grow up, take responsibility for our lives, and emerge from whatever test this planet has in store far wiser, more capable, and mature."

Saturn in Capricorn: "With the help of Saturn at maximum strength, your judgement will improve with age. You advance in life slowly but steadily, always with a strong hand at the helm and an eye for the advantageous situation. (EW flashback much?) Negatively, you may be a loner, prone to periods of melancholy. (but that's what makes Trowa, Trowa)

I'm excluding the remaining planets because they're to hard to connect in this summary.

Thank you, I hope you enjoyed the insanity. ^_^