If Sylphiel were a real person would she make for good marriage material? An amusing idea indeed. ^^
Looks: Lovely. Sylphiel is indeed beautiful, with a youthful and innocent appearance. Her gentle smile tends to make her catch and captivate one's attention and her voice is very soft and feminine, added with expressive green eyes and a delicate figure. And her smile often brightens her features like the rising sun.
Smarts: Intelligent. From her refined manner of speaking, plus the amount of books seen around her house, one can see that Sylphiel was brought up to have a good education. She possibly mainly studied the arts of poetry and also Theology since she is a shrine maiden.
Personality: Sweet, gentle, and kind. Sylphiel seems to be the type of girl who has been raised to be a proper lady. Her personality is reserved and she's very shy, especially when it comes to her feelings for Gourry but she'd risk her life for her friends and selflessly take the burden to help others.
Cooking: Great. Since the only family she's had for so long was her father, having to take care of him and help him for years has made her good with chores around the house, especially in the kitchen apparently because Gourry always raves about how good her cooking is. So there will always be home cooked meals with her around.
Fighting Skill: Fair. As a healer, Sylphiel can take care of many wounds at once, more so than one who doesn't know white magic. She is rather inept at combat, but she makes for good defense and if she needs to protect or help those she cares about, she could even use some black magic spells.
Special Abilities: Good. Sylphiel is said to be the human of the group that is the best skilled when it comes to white magic and divination. Her main area of expertise is in white magic especially with healing spells such as Recovery. She has improved on her flare arrows and can also cast the Dragon Slave.
Competition: Gourry. Plain and simple. He won't challenge anyone for her hand, since he is more or less oblivious to her affections for him but Sylphiel is quite smitten with him and won't bat an eye at anybody else.
In-Laws: An uncle in Seyruun and her father Eruk who is deceased. Not much is known about her uncle so it's hard to say if he would be an obstacle but Eruk certainly isn't. While he may be dead, he has always thought of what Sylphiel wanted so he would have supported whomever she would have chosen.
Economics: Fair. Since Eruk was a priest and a noble, Sylphiel has probably inherited a nice sum of money in the event of her father's death, but she can also make money from healing people so currency shouldn't be that much of an issue with her.
Sex: Interesting. Admittedly, Sylphiel will be nervous in her marriage bed being more than likely a virgin, but if her lover is caring and gentle then she will become more comfortable. Possibly even open to "ideas" that would make her into a real sex kitten but only if the love is there between her and her partner.
Overall: Quite a catch. If you can work around Sylphiel's feelings for Gourry then you'll have a loving and devoted wife on your hands who'll cook your favorite meals and shower you with her love and affection after you settle down with her.