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Arthur Penhaligon is the protagonist of The Keys to the Kingdom series.
Lord Arthur Penhaligon, Rightful Heir to the House, the Keys of the Kingdom and the Architect, Master of the Lower House, Lord of the Far Reaches, Duke of the Border Sea, Overlord of the Great Maze, Commander in Chief of the Glorious Army of the Architect, Master of the Middle House, Ruler of the Upper House, Lord of the Incomparable Gardens, and the Secondary Realms.
Arthur, a 12-year-old boy, has recently moved to a town and wants to fit into it. After suffering an asthma attack, he is saved by a mysterious metal object, called a Key, given by an even stranger character, Mister Monday, whose fellow Denizens bring an incurable plague to Arthur's town. Arthur hurries to the House, a mysterious structure that only he can see. Shortly after arriving in the House, Arthur discovers the structure of the house is a complete universe and is informed of his duty to unseat the seven Trustees who run the House, claim their Keys, and rule all of Creation. Arthur cannot live an ordinary life unless he overthrows all of the Trustees, who are also known as the Morrow Days. To do this, however, he must use the Keys, which infect him with sorcery and make him a Denizen of the House; and whenever Denizens appear in the Secondary Realms (everything in Creation that is not in the House, including Earth), they are inimical to mortal life. This dilemma is a constant theme in the books: as Arthur does not wish to turn into a Denizen; he often resists using the Keys, and only does when it is absolutely necessary.
Arthur's current title as the Rightful Heir of the Architect is "Lord Arthur Penhaligon, Rightful Heir to the House, the Keys of the Kingdom and the Architect, Master of the Lower House, Lord of the Far Reaches, Duke of the Border Sea, Overlord of the Great Maze, Commander in Chief of the Glorious Army of the Architect, Master of the Middle House, Ruler of the Upper House, Lord of the Incomparable Gardens, the House and the Secondary Realms".
In chapter one of Superior Saturday, it is implied that Arthur's use of the Fifth Key at the end of Lady Friday has completed his permanent transformation into a Denizen of the House: he grew taller and grew longer hair, looked better, and got stronger. Leaf and Arthur are returned to Friday's temporary hospital on Earth. Where Arthur starts to look at the crocodile ring on his right hand, which measures his metamorphosis from mortal to Denizen, he stops himself at the last moment, as "In his heart, he knew the answer without looking at the ring". This is confirmed when Arthur is now shown to have golden blood, and when he endures many physical attacks that a mortal would not survive as well as self-healing powers. His transformation also came with a negative side effect: his mind is sometimes plagued by thoughts normally associated with a superior Denizen (i.e. arrogance, contempt for those weaker than himself, anger, etc.), and he exerts great mental effort to suppress these thoughts. By the end of Superior Saturday, it is indicated that Arthur is at least 75% immortal, though it is hinted earlier that he is not necessarily a Denizen but may be something else.
During Lord Sunday, it is revealed that Arthur has been completely transformed by the Keys, but not into a Denizen. He takes the form of the New Architect during the closing chapters, suggesting that the Architect is not a Denizen. Another, mortal part of Arthur (created by the New Architect and thus a part of him in a similar manner to how the Old One was created by the first Architect) is created, who has Arthur's mind, memories, and emotions, goes back to Earth with Leaf, satisfying his need for 'a normal life'. This Arthur is referred to as 'Arthur' after this part of the story, while the 'other Arthur' is called the 'New Architect'. Notably the New Architect is said to be 'lying, for his own good' when he tells 'Arthur' he is now mortal, proving that Arthur is still immortal and possesses immense power. The human Arthur possibly serves a purpose similar to the Old One.
In the epilogue, it is shown that the New Architect has recreated Suzy Turquoise Blue (memories, emotions and accent intact) and is offering her the position of the new Sunday. Notably it is shown that the New Architect can change his appearance here; while talking to Suzy he takes the form of Arthur as he would at age twenty-one and in clothes from his original home. Suzy accepts the post, taking the title 'Lady Sunday' and also expresses a wish to 'grow up a bit', as she has been a child for at least two thousand years. The New Architect complies, and shows Suzy she has been recreated in an adult body, possibly a beautiful one, as Suzy is very impressed by her new countenance. The New Architect tells her that they need to recreate the House and re-populate it with Denizens. Suzy agrees and proposes the recreation of her friends Giac, Fred Initial Numbers Gold and Dr. Scamandros. The New Architect agrees and says they can help with the reconstruction.