XGaster

XGaster is more of a mysterious character, showing his isolation to the main characters except in special times, such as when XTale!Sans attempts to murder XTale!Frisk on stage. He's also a very clever man; knowing that everyone would soon revolt against him, he took his own soul and handed it to Ink for safekeeping. Despite this, he shows much more borderline sociopathic and abusive tendencies towards his children, seeing them more as "experiments" than anything else. He barely cares about anyone's else feelings, and only wants them to fall into his own hands, using them as a puppet to his advantage.

He is also shown to be extremely obsessed with control and shows several traits of narcissism such as ranting about his kids for fighting back and accusing them of being "ungrateful." He also calls the other universes that aren't his own "garbage." This all shows that beneath his stoic and calculating demeanor, he is mentally a child who sees his universe as a toy box.

The animated prequels display that he was once a genuinely kind being who simply wanted to make a perfect world for his children. However, as time went on, he shed every redeeming quality he once had and became more cruel as he grew more desperate for his "perfect world." He even displays several signs of outright sadism, finding the anguish his sons had after he killed Asriel and Asgore "exciting," and after brainwashing Frisk into a killer, mocks Chara, saying "Frisk wants to play with you." The closest thing XGaster has to a redeeming quality in the modern day is his friendship with Ink, but even then he isn't above manipulating him.

In XTale IX, he seems to briefly show a soft side again after interacting with Papyrus, saying how not having the power of the world gave him a new view of it before mentioning that he "hated it" and saying how he had gone too far to stop now, going back to his old self.

After XGaster's Future vision shows him the events of 0.4 as well as what leads him to it, he becomes obsessed with ensuring those events will happen. He also shows quite a bit of determination, as his goals are regularly shown to be tiring and exhausting yet he continues working toward them.