Nimue (NEE-mu-ay, NEEM-way, NIHM-oo-ay) is an Arthurian baby name and just one name given to the Lady of the Lake, also known as Niniane, Vivian (multiple spellings), Evienne, or Nivien. Some have claimed she is, or represents, a triple goddess, due to the fact that her names could come from the goddesses Coventina, Nemain, and Mnemosyne (and the Celtic love of triple personifications is well known).
Nimue may be a corrupted form of Nineve, which may have been taken from Nineveh, a city in Syria, which is an Assyrian name that ultimately means "a habitation of rebels." If not, it may be taken from Mnemosyne (meaning "memory") a water nymph from Greek mythology whose story was a bit similar. As Nimue could be a spelling error, along with Nynyve, Nynyue and Ninive, there is no set-in-stone pronunciation. In Malory's Le Morte d'Arthur there are three different spellings used: Nimue, Ninive and Nineve/Nyneve (perhaps on purpose because all good things come in threes). In Tennyson's Idylls of the King she is Vivien. Vivien(ne)/Vivian(e), although its own name, may have been inspired for this character by the Celtic water goddess Coventina and have no relation to Vivian of Latin origin, which was originally a unisex and/or masculine name meaning "alive." Her name as Niniane most likely comes from the Celtic word nino, meaning "ash" but it is so close in sound to Viviane that one must wonder which came first.
The Lady of the Lake is world famous for giving King Arthur the sword Excalibur, but her story is deeper than that. She was foster mother to Sir Lancelot, raising him underwater. Before that, Merlin met her and fell so in love that he agreed to teach her all of his magic. She was, for a time, his scribe, student and lover, but when she grew more powerful than Merlin she imprisoned him and may have taken over his duties to King Arthur, which may have led to Arthur's downfall. Yet, she was one of the three queens that escorted Arthur to Avalon. Every author shares his unique version of the story.
Nimue has always been a rare baby name, only given about 30 times in the U.S. Ninive has only recently had some use in the U.S. starting approximately 2004 just a handful of times a year. Niniane, Nyneve and Nineve seem to be the most elusive spelling choices. Vivian has always been popular, given to 2629 girls in 2013 which gives it a rank of #119 (Viviana is #445, and the French spelling Vivienne is #280). With the e at the end, however, Viviane and Viviene are much less popular options, with only 12 girls given the name Viviane and 7 given Viviene in 2013. Vivianne is somewhere between the two, not ranking but given to 75 girls in 2013