Neptune doesn't rule any zodiac sign in traditional astrology. In modern astrology, it rules Pisces.
Astrologer Judy Hall describes Neptune as "embodying the urge to transcend the boundaries that keep the soul separate from the whole. It is the urge to transcend the everyday world, to move into higher states of consciousness and to merge with the divine or to escape reality".
Keywords for Neptune include dreams, imagination, inspiration, clairvoyance, escapism, confusion, and illusion.
A good place for Neptune is Pisces. A well placed Neptune brings inspiration and is a visionary. A bad aspected Neptune tries to escape reality.