The character of Neptune is based around his relationship with the objects in the Kuiper Belt and beyond. The planet's tendency to become perturbed and expel objects that come within its gravitational reaches seemed to me to be a good place to start developing the character of the eighth Planet Club member and Pluto's all-important next-door neighbor. It's not surprising, then, that Neptune quickly became a sort of villain whose hostile and erratic response to trespassers was feared by an entire community of KBOs and SDOs. Employing his slingshot or catapult (for those heavy-duty, mass evictions), the Neptune character hurls space objects "where the Sun don't shine." The degree to which his actions are forced upon him by nature (He's been endowed with great size and potent gravity), or are evidence of more nefarious intentions, is open to debate.
The planet Neptune has some rather interesting characteristics and features that I thought could easily be "mined" for character development. It's an intensely blue and windy place with a notable dark spot where terrifying, electric storms rage. Liquid diamond abounds on Neptune (though it is true that other gas giants experience this phenomenon), and hard diamonds fall from the sky. So, naturally, Neptune the character became a diamond hoarder,obsessed with maintaining his diamond production and keeping his operations under wraps, a kind of "Uncle Scrooge of the Solar System." This is something Pluto discusses briefly as he describes his experiences in the Planet Club and the interesting planets who made up the Club.
There are a lot of characters in "The Pluto Diaries," and most of them are not central to the plot, the true story of what happened to Pluto in 2006 and the events leading up to it. As a result, it wasn't possible to fully explore the depths of these characters, and there are definitely characters that would be interesting to develop further. Neptune is one of those characters who got a little more development because of his proximity to the Kuiper Belt and the issue with Pluto sneaking into his orbit every 20 years of his 248-year orbit. Still, I could see going further with Neptune. Perhaps in another story?