We learn Seeu's story entirely through Take. Take was a college student in the 1940s who somehow managed to fall through a dimensional rift into the Second World. He managed to befriend both Seeu and the other people of the Second World.
Seeu was born alone. And as it turns out, it was a rather good thing that was true. Seeu was a prince of the second world. His mother was sick, and very weak, so his father had the palace isolated from everything else. He himself had to take many precautions to prevent germs from the outside coming into the castle. She was cared for by 'dolls,' the mechanical creatures used as servants. Seeu's mother passed away during childbirth, leaving Seeu alone in the castle, tended only by the dolls. He never once set foot outside.
So he ruled his people from inside the castle. But he had never been touched by another human being.
"I cannot say that I grew up to be a healthy human being. I... It had been predicted for a time now that I would not grow up 'right.' A living creature that was never once touched by a human hand...will neve accustom itself to humans. My mother's remains were cleared away by the dolls, it seems. And for fourteen years... no living person... or rather, no living thing, has ever...touched me."
But the the Second World, Asu, hides a terrible secret. Every one of the inhabitants of this World are infected with a wasting disease that wears away at the joints. Except for Seeu. But was Seeu all that 'healthy' anyway?
"During the day, he truly shows such a peaceful, kind face. But ... there are times ... at night - the mad prince..."
It is because of this that the people of Asu are obsessed with finding a way to transfer a human soul into a 'doll.' If they could do so, then the people would be saved from the disease.
No solution is forthcoming. As the population of Asu steadily drops, more and more people decided to attack the castle. They tried, but couldn't break the barrier surrounding it. Seeu honestly didn't seem to care whether he lived or died.
Eventually the people become resigned to their fate, and a sort of wistful happiness sinks over them.
"Even if they aren't eternal...flowers are beautiful. This planet, too. And...it's people."
Eventually only Seeu and Take are left. Seeu expresses the fact that he really doesn't care if he lives or dies. Take, slowly dying on the other side of the barrier, is vehement that Seeu must live.
"Prince Seeu! Have you ever found something that made it great to be alive?! You're not alone. And if you live, then it means that Asu lives. You said so yourself, remember?! The royal blood runs thick. You just can't die thinking like that. You said it, and so did many others. We all will eventually die. Well, of course we'll die, but it doesn't mean that we should just sit and take it! If there's even one thing that makes you think, "I'm glad I was born..." then you've already won. I want you to win. I don't care if I'm being selfish...but I just want you to win!"
Later, as Take is finally dying, the Lunar Mercury activates. It slices through Take on its way to Seeu, and Take manages to transfer himself into a 'doll' just before he dies.
Seeu accepted the power of the Lunar Mercury...
The later events of Seeu's life are much more muddled. Apparently he was in a group of people who held the other living weapons, and they fought in some sort of war. From what I can gather, Seeu was especially close to Kagami. After the war, this group splintered; Kagami and Lagunahaan apparently dead, with Kura and Idou still in alliance...
But Emperor Kura and Idou now seem to be fighting against Seeu. Seeu has made a 'doll' of Kagami that follows his commands. There are several scenes that point to the 'doll' of Kagami actually being Kagami or maybe carrying some of his memories, but everything is still very muddled. Seeu's main objective, although this is unclear at the beginning of Planet Ladder, is to protect Kaguya (the main character girl) from Kura and Idou.