Neptune represents intuition and dreams: what in you goes beyond the rational — artistic inspiration, spirituality, empathy, but also haziness and illusion depending on context. It moves very slowly (about 14 years per sign): like Uranus, it marks a generation above all, the house it occupies personalizing its individual reading.
Discovered in 1846, Neptune does not belong to the set of seven classical planets either, and therefore has no essential dignity in the Hellenistic system, for the same historical reason as Uranus.
Modern astrology conventionally assigns it co-rulership of Pisces, a sign previously governed by Jupiter alone — a widely shared convention, postdating classical dignities.