Neptune is considered to be weird, random, and does not have great memory. This is shown when he meets Pluto again after a very long time, because he does not seem to remember the Dwarf Planet. But right after meeting him and having him introduce himself again, he forgets everything again, causing Pluto to get desperate and go hang out with the other Dwarf planets instead. Neptune is also shown to be untrustworthy, where right after promising not to tell the rest of the planets that Uranus stinks, everyone was told about his secret shortly after Venus asks "Tell us what?". Neptune then brushes it off, not acknowledging how angry Uranus was. In "Why is Neptune MISSING!?” his personality was revealed to be caused by the insanity from loneliness for 165 years because of his orbit, leading him to make imaginary friends out of asteroids in the Kuiper Belt, notably Guillermo. Neptune is considered as being the comic relief character due to his personality.

However, Neptune’s memory and sanity seem to be steadily improving. In The Moons of Uranus, he remembered that both him and Uranus had moons, he shows sympathy towards Uranus’s moons, and even lectured him about scaring them away. Neptune also offered to look for them with him and suggested Uranus to apologize instead of acting like it was nothing. Another example is how Neptune memorized all of the names of his moons in The Moons of Neptune - the only gas giant to do so.