At first, Seeu comes off as a villain. This is easy to see; he's quiet, he's dark, and he doesn't appear to care too much about the consequences of his actions.
However, later events reveal the true depth of Seeu's character. Along with his tragic past, his best friend died in his place in order to change the flow of history. Idou implies in the first volume that it was Kagami's death that truly unhinged Seeu. Even though there had been hints of his insanity in the palace on the Second World, it wasn't until Kagami died that Seeu went off the deep end.
Currently, Seeu cannot even hear Kagami's name without "falling asleep." He uses this mechanism whenever his emotions overload; he just falls asleep instantaneously in order to forget about the pain of the past. Although other people have prompted this response, Kagami is also the only person who Seeu has built a doll of.
The doll is another element of Seeu's character. It is easy to see why Seeu would be obsessed with making a working doll; his planet died because doll technology wasn't perfected before everyone died.
But it is impossible to put someone's soul into a doll posthumously, isn't it? At least, according the Emperor Kura, once a person is dead they are beyond retrieval. Then why would Seeu build the dolls of Kagami, unless of course he knows something the rest of us don't...?
The doll of Kagami displays some weird behavior as well; once, it seemed to actually be Kagami; however, that episode soon passed and it returned to its taciturn state.
Even Seeu seems to view the doll as only a doll; he commands it around, he orders it to do things, he says that it isn't a person. But some part of Seeu that isn't repressing the memories clearly wishes that Kagami were alive again.
Seeu continues to fight Kagami's battles; he works with Meshie Laracott and Gaviela in order to halt the end of the universe. He had kidnapped Kaguya for this reason.
So, in all, Seeu is one of the "good guys." However, his unbalanced nature makes him dangerous, to himself and others. But with Kaguya's innocent friendship and the possibility of Kagami being alive... Seeu could eventually recover.
The problem with analysing Seeu's character is that so much about Planet Ladder is simply unclear. However, the next tankouban is coming to the U.S. later this year, so we'll see what happens then.