Planet X

Planet X is outspoken, confident, sarcastic, and highly expressive. He tends to approach others with an informal, almost cocky friendliness, frequently using slang, teasing people, and turning tense situations into jokes. He is competitive and enjoys feeling impressive, particularly after returning to the Solar System, where finally receiving recognition after billions of years alone briefly goes to his head. His humor can also become cutting: he mocks Neptune, trades insults with PSO, and has little patience for people who evade his questions.

Beneath this swagger, X is extremely sensitive to rejection and exclusion. His exile left him with a deep fear of being unwanted, and much of his early behavior is driven by the belief that the Solar System formed a family without him. When speaking honestly to Wise, X admits that he does not feel as though he belongs with the other planets and wonders whether he would be more comfortable simply remaining in the darkness. His attachment to Guillermo similarly reveals how badly he needs companionship; despite knowing that Guillermo is only a rock through which he talks to himself, X treats him as emotional company.

X also possesses a powerful sense of justice. Even before his exile, he objects to Jupiter's reckless orbital migration and refuses to help conceal the destruction caused by the Grand Tack. His downfall comes specifically because he insists on telling the Sun the truth. However, isolation twists this moral outrage into fixation. By the time X returns, he has spent billions of years concentrating on revenge, and he initially interprets almost everything through the lens of what Jupiter, Saturn, and the Solar System owe him.

This produces X's greatest character flaw: when he believes himself morally justified, he can become vindictive and self-righteous. After replacing Jupiter, he begins making retaliatory demands and eventually attacks the moons during his challenge with Ganymede. He rationalizes the harm by pointing to everything he endured, only to be devastated when Proteus tells him that being hurt does not justify hurting others. That confrontation finally forces X to recognize that he has started reproducing the same behavior he condemned.

From that point onward, X becomes noticeably more reflective. He remains sarcastic, stubborn, impulsive, and confrontational, but he increasingly judges situations by whether his actions actually help people rather than whether someone "deserves" punishment. He ultimately concludes that his past will not define him and decides that he wants to be useful to planets who have no protector of their own. By Season 6, he is able to recognize the same revenge mentality in the rogue planets and warn them that vengeance produces no real winner.