A common problem with fanfiction as a whole is that people never seem to get the characters they're writing right. Usually it's because the main focus is on another character so they become lax with other characters, particularly the minor characters. Since I've seen this so much in fanfiction where Sylphiel has a small role, I thought to make a section on how to write Sylphiel and make her in character for future fanfiction. If you have a difference of opinion, please feel free to comment about it and I'll add it in. Overall, this guide is simply made from my own experience of writing Sylphiel in the past as well as merely thinking of what she would do or say in different situations and how I identify with her as a character.
Dialogue - For the most part, Sylphiel is very formal when speaking to other people due to her high class status. Even if she is very close with whom she is speaking to, she always attaches a Mister or Miss to their name (-san if you wish to use the Japanese alternative). With her father and Gourry, she deems both with high regard as shown by her use of "Otou-sama" and "Gourry-sama" in the Japanese sub version. In the English version, "Gourry dear" is used as a way to show how affectionate Sylphiel is towards him (Note: in the novels she always refers to him as Master Gourry, which is a more appropriate title for the particular medieval setting Slayers is set in). She calls Eruk "Father" and never anything else; she'd never use "Pop" or "Daddy." Sylphiel can also be seen speaking in a concerned or uncertain manner, depending on the type of situation she is in.
Tone - Sylphiel has a very quiet, soft-spoken tone when she speaks. She rarely ever raises it unless she is performing an incantation or is overreacting to a situation such as latching onto Gourry when we first meet her. When she is angry, she talks in a tone of defiance and will spit out a person's name if she extremely dislikes them as seen in the novels when she speaks to Kopii Rezo. If she is upset, she speaks in a wounded tone of voice because she is quite sensitive and is hurt easily.
Inner Monologue - The few scenes where we see Sylphiel's inner monologue were used mostly to clarify her actions and help explain certain questions that cropped up, such as why Kopii Rezo was trying not to go near Flagoon; it was from Sylphiel's inner monologue that we learn he was afraid of the tree and she then deduced it as the key to stop him. From the way she acts overall, one can tell that she thinks a lot to herself as she observes her friends and what goes on around her.
Movement - Since Sylphiel is mostly associated with the upper class, it is her nature to be very proper in her movements and mannerisms, but she is also very affectionate and caring. Depending on what version you want to work on, she can either be seen bowing respectively to those she holds with high regard, or hugging them on sight (i.e. Gourry). There are a few times where she over exaggerates in a situation, mostly characterized by fainting. Then there are the moments where she shows how girly she really is when she becomes excited over cute, adorable objects.
Association - Although Sylphiel is reserved, she is still quite friendly with those she meets and rarely thinks of people as mistrusting until they show their true colors. She is also exceptionally close to Lina and the gang, despite the appearance that she is a loner; this is mostly due to the fact that she tends to be more of the stay at home sort. There is the rare occasion where she will decide to travel, mostly to be alongside Gourry or to help others if they are in need but she's not one to go out and do this or that without a well enough reason.