As I speculated earlier. I believe Sylveon is a "part 1" in the series of two new eeveeloutions. This is quite the long shot but I really think Sylveon is either a Normal type pokemon or a completely new type (like light or love) But I ALSO believe Sylveon is FEMALE only. Meaning if your Eevee is a female, it could evolve into this under certain conditions. This would make sense seeing as the prefixes "sylph" and "nymph" (from Ninfeon) are FEMININE prefixes! I took Latin for a little while so there's my credibility I guess. The Pokemon also has very obvious feminine features and attributes.
Only major issues with this is Neither Sylph nor Nymph have female specific origins. Neither have real gender and can appear as male or female. Secondly, NINfia and SYLVeon don't give us NYMph or SYLPH. Pronunciations are different, letter build up are different. SYLV as some pointed out can come from Sylvia which is feminine and latin that revolves around forests. But at the same time there are names such as SYLVester which is male so name origin is a nearly useless discussion without more info on the Pokemon. NINfia by many was thought to relate to NINja. Its missing like 1 syllable but still, the names cannot give us a decent enough meaning as pronunciations are too different to be a close coincidence.
When we get actual typing and Pokedex history, all the background info collected so far will either change completely to something different that what we already have, or solidify one of the hundreds of different meanings we already have.
As I stated earlier I do think this is just a "part 1" and I think maybe we'll see a male counterpart very soon. There is no male prefixes that begin with S from Latin, just to throw that out there.
I think the male version would evolve from Eevee very similarly. And have more of a blue/black color scheme.
Sylvester is a Male Latin name that has roughly the same meaning as Sylvia. Sylvester means wooded. There are other latin words that start with S that are masculine terms. Quick google search brought up five different dictionaries with a long list of words.
Also backing up my theory, we HAVE seen plenty of male and female exclusive Pokemon come in pairs!
Of course all of this is just speculation. But it would really be cute and it makes sense to some extent.
There are many feminine looking Pokemon as part of pairs that are not 100% female.
*bulbagarden.net image removed**bulbagarden.net image removed*
All Pokemon with split gendered evolutions have a 50% chance of being Male or Female.
Snorunt, Ralts, Burmy. These are the only ones with split evolutions with one(or more) being a specific gender. Snorunt is 50:50, Only females can evolve into Frostlass while either female or male can become Glalie. Ralts is 50:50, Only males can evolve into Gallade, while either male or female can evolve into Gardevoir (most feminine Pokemon ever).
Burmy, while it has a 50:50 chance of being either evolves into two gender specific Pokemon. Males can only become Mothim and Females can only become Wormadam.
Only other Pokemon in existance that has a gender based evolution is Combee, but unlike the others ONLY females can evolve so Combee is a conditional evolution.
As for how Sylveon looks, that's all personal opinion. To me Sylveon is no more girly than Vaporeon, Espeon, Flareon, Glaceon, and Leafeon. To me Jolteon and Umbreon are the only two Male Eevee Evolutions. There are a lot of pictures on Tumblr of male Sylveon Gijinka as well, so Sylveon looking female only is personal opinion.