Yes it is. We don't really use the diaeresis anymore, but it is still accepted if you want to use it. In English it isn't used it for accentuation, but for proper pronunciation. A diaeresis is placed over the second of two adjacent vowels, and indicates that each one should be pronounced instead of combining them into one sound. Like the word naive, no one writes naïve anymore, but it is acceptable to write naïve. The diaeresis stopped being used in English (well, American English) in the early 40s for some reason, so you don't see it much anymore except for words that we borrow from other languages and proper nouns, and even then it's really rare. But, if you wanted to be very technically grammatically correct, you would write coöperate, reënact, noöne, etc etc. Read the New Yorker and you'll see a diaeresis every once in a while (the editor is a stickler for that kind of stuff).
I've never seen it used properly unless it was on an imported word. Thanks for the info.

wow, i never knew....
thanks for all the help, we know 9, maybe 10 (saix's name)
anyone know the other 3?
does it show marluixa's name?
It only shows OXIII members I-VI, and XIII, in KHII.
Marluxia is number XI. So, therefore, no, at least, not by canon. A grand total of 7 members covered by the game itself. All else is fan fiction or speculation.

I do however think that names are restircted by gender, since there are differences between gender in the Nobody organization. Larxene is visibly female, as are most of the others, but they have male voice actors, except for Larxene. Even then, her title is one that isn't a very enlightening one. Being called a Nymph is not a good thing. So I doubt that Axel would be Lea, but anything is possible.