Anyone with a basic knowledge of the principles of astrology will know that there is so much more to a person's character than their sun sign. And since Kanzaka-sama did not give Sylphiel a birthday, it is hard to say what her sign really is but I will do my best. ^^
For the most part, Sylphiel fits the physical qualities of a Pisces perfectly. Pisceans usually have gentle eyes and unstyled, natural-looking hair. The eyes are a dominant feature in many Pisceans, and will probably appear to be gentle, perhaps even showing a capacity to be very emotional and their hair is in its natural state. Sylphiel's most prominent feature are her wide, soft green eyes and her hair is long and unstyled, like the description indicates.
Pisceans are selfless, spiritual and very focused on their inner journey. They also place great weight on what they are feeling. Emotions define Pisceans, and it's not uncommon for them to feel their own burdens and joys as well as those of others. The intuition of the Pisces-born is highly-evolved. Truthfully, the most notable thing about Sylphiel is her spirituality and the way she is more empathetic and sensitive to everyone else. She sensed Lina's true feelings for Gourry no matter how much Lina pulled off she just wanted the Sword of Light.
Pisceans are compassionate, charitable and will quickly put the needs of others ahead of their own. It's this kind of self-sacrifice that keeps Pisceans going. The flip side to their giving natures? The oft-timid Fish are likely to be taken advantage of by less well-meaning souls. Sylphiel herself puts others' needs ahead of her own and can sometimes have her kind gestures taken for granted.
At times, Pisceans can have difficulty distinguishing fact from fantasy - they tend to get caught up in their dreams and illusions of how things should be. This is true of Sylphiel since Prince Phil was nowhere close to her image of a gallant prince. ^^;
Pisceans who fear that their pleas are not being heard tend to lapse into melancholy, or a kind of pessimism that can lead to procrastination and lethargy. At times like this, Pisceans take some time for themselves, in order to peace and quiet again. It's noticeable that Sylphiel tends to be a loner herself, one example being the way she secluded herself after Eruk's death and when she said she had played in Flagoon's underground caverns by herself when she was little.
When it comes to relationships, Pisceans can be very romantic, dreaming up delicious treats for their lover. *cough* Sylphiel -loves- to cook! Although they view the world through rose-colored glasses when they are in love. Pisceans are generally gentle and relaxed people who are on the shy and reticent side. Sylphiel is incredibly shy herself and has trouble telling Gourry how she feels about him.
Colour can be a comfort, and for Pisces, purples and soft whites are the most relaxing. This could be a tie-in with what Sylphiel's clothes represent along with her spirituality since she wears a lot of purple tones.
When it comes to the game of love, Pisceans are caring and romantic and a most creative lover. Now I know Sylphiel would be entirely devoted and loving to her lover but she may be a bit meek in the bedroom -- though who is to say that she wouldn't come up with a "few" ideas.
The great strength of the Pisces-born is their compassionate and charitable nature. They love to help others, and do so in the most imaginative of ways. It is their compassion and empathy that win people over. Sylphiel herself is loved for these common traits and make people love her and want to protect her easily.