Nichothodes Euthandor Timosus Benigedy Koryfoides, otherwise known as Nick Mallory, is the brother of protagonist Maree Mallory in Deep Secret and is himself the protagonist in The Merlin Conspiracy. In the former book he is about 14 or 15 years old; in the latter, he's about a year older.
In Deep Secret, it transpired that Nick -- born and raised on Earth by stepfather writer Ted Mallory, and mother Janine Mallory -- had his own secret origin story. Janine turned out to be a refugee from the Koryfonic Empire of worlds, and her son Nick the heir to the throne on one of those worlds.
In the course of that novel, Nick and his cousin Maree Mallory became entangled in a plot involving the Magid assigned to the Empire, and Nick became fixated on wanting to somehow get the training necessary to become one of that hidden clan. He turned down the throne and kept trying to find a way to do that. In The Merlin Conspiracy, it looks as if Nick might be able to achieve his goal.
Nick starts his journey at a writer's convention, where Ted Mallory has dragged him along to meet one of Ted's favorite authors -- detective fiction author Maxwell Hyde. While standing in a hallway as his father exchanges pleasantries with Hyde, Nick finds himself jostled by someone, and suddenly realizes that he must have fallen asleep, because he seems to be having a dream. It's only after going through a series of confusing and taxing experiences that Nick realizes it's not a dream after all -- he's found his way into a path between alternate worlds and doesn't know how to get back.
While wandering in the dark pathways, Nick encounters a drunken magic-user who gives him a witchlight and tells him the secret of getting through the dark paths and into another world. The magic-user then stumbles off leaving Nick alone again. The secret is that apparently you have to help three people before you can escape the paths.
Nick proceeds to encounter his major allies in the paths -- the first one he helps is Mini the lost circus elephant, who he helps to turn around as she panics in a tight spot between stone cliffsides. Nick can hear her voice, and she can hear and understand both his thoughts and his speech.
The second person he helps, or promises to help, is the other protagonist in the book, Roddy (Arianrhod Hyde), who tells him about the conspiracy threatening her world, and asks him to help her fix things. Nick promises to try.
Nick's third major ally turns out to be the drunken magic-user. Their alliance helps Nick rescue Romanov, meet Roddy's relatives and later Roddy herself, and eventually to get into a position to be able to fight the conspiracy together.
Nick was evidently based originally on writer Neil Gaiman, who was very close to Diana Wynne Jones. She gives the character a habit of waking very slowly in the morning, such that he can't talk or deal with people until he's had several cups of coffee. This affects the plot in various ways in both Magid novels, and helps to make Nick both memorable and likeable. Nick also shows both loyalty and kindness, despite his youth. He's always drawn to living beings who are good and decent, rather than the phonies or bullies of the worlds he finds himself in. The charm of his narrative style is one of the things that makes this book appealing. Nick's young and fairly clueless, but his basic decency helps him to land on his feet, whatever happens.