Planet X

When he was younger during the early days of the Solar System, he was initially shown to be a friendly, outgoing, & conversational ice giant, however his positive demeanour would quickly shift off the deep end when Jupiter treated him poorly and Saturn reciprocated it. Even at a young age, Planet X was shown to be a logical thinker & voice of reason, desperately trying to stop Jupiter from crossing over to Saturn's orbit knowing that it would cause irreversible chaos.

When Jupiter destroyed the original rocky planets by accident, Planet X was revealed to have a brutal sense of justice, wanting to tell the Sun the truth regardless of the consequences Jupiter would face and seemingly unable to forgive him. He also seemed to hold the Sun in high regards, claiming that whatever the Sun asked was "his right", and that the planets shouldn't defy him. This sense of justice has questionable motives sometimes, as when he attempted to reveal Jupiter’s actions he almost destroyed the new rocky planets. This would then result in his ejection.

He has been enduring solitude for billions of years, watching the Solar System from afar. It can be inferred that he heartily loves his lost home, namely the Solar System, so much that he spent all his time "watching & listening to everything that has been going on there", meaning he likely knows about "everything" that has transpired in the Solar System since the very beginning.

Planet X seems to have considerable mental fortitude considering he appears to have remained mostly sane in spite of his isolation for billions of years, whereas Neptune had lost his mind in an orbit far less remote than Planet X's. He claims to have retained his sanity by staying focused on revenge. However, it's apparent that preserving his sanity has come at the expense of eroding his sense of empathy and humility. He is shown to be even more arrogant and self-centred than his younger self, and can only hold transactional, superficial relationships with others at best. He has developed an unhealthy obsession with beating those he dislikes and ensuring they get what he thinks they deserve, which makes him come off as "unlikable" by the others. He also has developed some manipulative tendencies, like telling Uranus & Neptune that now they won't be neglected by others or pronouncing Uranus' name correctly, and even trying to manipulate the Sun into changing orbits as seen in What If We Replace Jupiter? - Part 3.

The only somewhat genuine friendship Planet X seems to have is with Proteus, who eagerly made himself Planet X's "first friend" on their way back to the Solar System. Planet X confided in Proteus about his worries of returning, and how he wasn't sure the other planets would be happy to see him or if he'd be able to make friends having spent so much time alone, but Proteus allayed his concerns and explained what friends do. It is currently not evident if Planet X values this friendship with Proteus or if he took his words to heart, as Planet X still remains as unforgiving as ever in spite of Proteus promoting forgiveness as a hallmark of friendship.

In "The Battle of Planet X! - Part 4," Proteus tells X that he was so bent on revenge that he didn't actually try to communicate with Jupiter, and instead hurt so many moons. All of the planets were scared for their moons. "Caelus" got mad at him, and Proteus gave him a harsh talking to, which convinced him to leave for good after talking to Jupiter. He says, "There are plenty of Solar Systems that don't have a- a Jupiter to protect the smaller planets. Maybe I can- be that. somewhere else," showing that he had a change of heart and decided to leave and protect another system elsewhere.

Abilities Planet X seems to be quite good at dodging asteroids and going by himself in space without any huge gravity sources to hold onto. He also has a very good hearing despite not having ears.

idk