In my head he's a silly little alternative goober and I totally do NOT project my own mental illnesses onto him.
But in all seriousness he's genuinely just a silly guy. I don't think he's hiding or faking anything relating his actions or personality, how he acts is true to himself and that's that. He doesn't take anything that seriously and is very honest and kind. I don't think there's any mal intent in his forgetfulness, even if it unfortunately does hurt others sometimes.
I headcanon him as autistic from the way he acts. It's not just because he's silly but there are actually several factors as to why I headcanon this. Then again maybe I'm projecting lmfao. Also because of this I headcanon human him would use a lot of physical gestures when talking/explaining/showing his emotions, just like how he changes shape in canon.
The DID headcanon is because it just makes sense to me: For example the 'ordered by size' episode and the 'how did neptune get his moons' episode. There have been many cases where he is shown to have memory loss or not remembering certain things that he proceeds to know on a later date, which is something that can happen from DID.
I think he'd idenitfy as aroace and agender because he doesn't care and doesn't have any interest in those things.
This is one of Neptune's alters (doesn't have a name so I'll refer to him as Neptunus (look at Misc. for context)). Compared to Neptune, he is more knowledgable and his knowledge resembles Jupiter's. This alter is triggered when Neptune takes over Jupiter's orbit, but only when he has actually orbitted and spent some time there (Passing it, for example, doesn't trigger the switch).
A lot of Neptune designs I've seen have him in pretty warm/winter clothes, and I definitely see the logic, but I headcanon he'd wear more alternative clothes. Because I also headcanon him as being agender, I imagined his clothes to be pretty androgynous. I gave him some wide legged tripp pants and a turtle neck with long sleeves (that also act as fingerless gloves). The tripp pants can have different kinds of decoration but I prefer the chains and 'suspenders'. Despite me headcanoning him as alternative, I still think he would dress pretty casually, which is why I went pretty simple with his clothing. His hair is obviously teased, but even without teasing it'd still be pretty frizzy and have a lot of volume (see picture to the right). 
He also has black eyeliner under his eyes. Like all my solarballs designs I used the canon colours/mapping for their skin; I only added light blue fingertips because I thought it'd be cute.
For some reason I made proto him dress emo even if I don't headcanon him to enjoy emo music. He really wants to keep the side fringe, but because of sensory issues it's very hard. He loves wearing (ripped) skinny jeans, but they have to be stretchy (again because of sensory issues). He loves wearing (studded) belts not because they're practical but because they're cool (me and you both buddy). For everything else it's all pretty casual and comfortable.
Because Neptunus has a British accent I decided to make him fancier. He keeps the tripp pants but removes the chains and extra accessories. He also gets a blouse, a waistcoat and a tie. I gave him some heels (that coincidentally resembles Uranus's). His hair is tied up in a bun, but two hair strands are much loser to resemble a cord. Like his canon design, I kept the small round glasses. He doesn't actually need those glasses though, it's just for show.
When I was shown Neptune for the first time, he reminded me of Izzy from total drama for some reason, and then I got shown the "What if planets were ordered by size" episode and my mind exploded. Izzy literally goes through a similar change in total drama; Something happened and suddenly she gained an alter ego that's super smart and also wears glasses. Because of this I sort of base that version of Neptune off of her.
The song "Every Snowflake Is Different (Just Like You)" from My Chemical Romance reminds me of Neptune for some reason. It's probably the silly positive vibes the song has. Also Neptune's open and kind nature adds onto it, because even though he wouldn't outright say it, the message of the song is something he'd promote I feel. (Extra: the lyrics "...blue skin, and the very silly way you grin." fits him a lot too.)
I'm against Neptune x Uranus solely because Neptune called Uranus his "cousin". I feel like the ship actually has a very cute set-up; They have been friends since day 1 basically and they're the closest with each other (literally and figuratively). Only because of the fact that Neptune called Uranus his cousin, I (and Neptune and Uranus themselves probably) genuinely can't see the relationship as anything more than best friends, and that's fine! Normalise letting best friends be best friends /lh /silly
As you've probably seen on this page, I associate Neptune with music and mp3 players. I think he'd have a white ipod nano to accompany him during his long lonely journeys around the sun. I also think it's very comforting for him to listen to music because his part of the solar system is very empty and lonely, and even though he has his moons this is an alternative to distracting himself. The problem is I still don't have any ideas about what kind of music he'd listen to.
He loves breaking the 4th wall for no reason. The reason I don't call Neptunus smart!Neptune or something, other than me headcanoning him being an alter instead of a different version of Neptune, is because of the breaking the 4th wall thing. Neptune's smart too, just in a different way. Neptunus also loves breaking the fourth wall though, for example the pulling out a laptop thing.
In the third big bang episode, you can see proto!uranus and proto!neptune playing with little asteroids. Because of this I headcanon Neptune having a life long fascination with asteroids (for example the entire thing with Guillermo and his other painted asteroids) that started during the beginning of the solar system. (Extra (kinda silly and a joke): his moons are basically asteroids that got caught up in his orbit and he also cares a lot about them.)
He refers to Earth as 'Terra' in the show, so I think this alter only knows all of the planets's Roman names. In the second part he refers to Uranus as Uranus (Greek) though, but then again it's pretty inconsistent since he loses the british accent too so I'll just imagine that never happened.
