In P. D. James's novel The Children of Men, Xan is the Warden of England, a dictator with absolute power, symbolized by his Coronation Ring. He is also the cousin of protagonist, Theo Faron. For a while, Theo serves as Xan's adviser. To outsiders, they seem "close as brothers," but Theo does not think so.
In fact, Theo does not feel close to anyone. He has detached himself from life, cut himself off from his family (his wife leaves him after the accidental death of their daughter), and even gives up on his profession as a history professor. While he may not be completely comfortable with the power and the violence that Xan uses, Theo doesn't have enough spark to do anything about it. He is more complacent than anything, dry and nearly despairing, unable to rouse himself to much action. He feels that sometimes people must "tolerate" some evils "as the price of sound government," but this seems like an excuse for his apathy. At one point in the story, he even leaves England for several months and travels in Europe, just wanting to get away from everything.
After becoming involved with Julian and the Five Fishes, however, Theo starts to come alive. He begins to realize the full horror of England's "system" of rule when he witnesses the Quietus and even lashes out to try to stop it. Theo goes to Xan to complain about this abuse, which he identifies as murder. He is waking up, coming out of his complacency. Xan will not make changes, however—not to the Quietus or to the mandatory testing. He merely tells Theo to be "sensible."
As the story nears its end, Theo finds himself acting in ways he never though he would. He goes on the run with Julian and the others, helps Julian deliver her child, and even kills Xan (who tries to kill Theo first). Ironically, though, one of Theo's final actions is to put the Coronation Ring on his own finger and assume Xan's role as Warden. He tells Julian that it will only be for a while so he can correct the abuses, but readers are left wandering if Theo has succumbed to the temptations of power and violence in the end.