Jonathan Creek is an extremely intelligent man who can wrap his mind around problems, and solve them in a few hours. As such he has found an ideal profession, designing illusions for magician Adam Klaus. And he comes in most useful when playing logic games.
Although Jonathan is a fairly quiet man, he will voice his opinion and won't be taken for granted by anyone. Although he will listen to people he will quickly dismiss ideas which he knows to be wrong. Thus to some people he can seem like "he's always right". As such he is open to people who want to take him down a peg or two, especially Maddy. He is therefore always on his toes and won't let the wool be pulled over his eyes easily.
Jonathan leads a fairly orderly life, and at first didn't like Maddy bringing chaos into his neat little world. He even voiced his opinion on the fact. However it would seem that a gap in his life has now been filled, and he has started to enjoy himself solving crimes. He has a hobby as it were, which he can pursue in his off hours. We can also see Jonathan take credit for the things he does. Something which he would appear to like, as Adam takes most of the credit for his illusions. Although having said that Jonathan does not crave the limelight, and is thus happy to take a back seat to Maddy, who takes the main credit, but does at least mention Jonathan in her work.
Jonathan isn't the best looking man in the world by any means, but he is immensely attractive to the opposite sex. Something which while he likes it, he isn't always sure what to do about it. What he lacks in looks, he makes up for in personality. He likes to make jokes, and see people smile. However he sometimes does find it a chore to always be entertaining. An example of this was when he first has lunch with Maddy, he knows even before they start that she will want to see a trick of some sort. Which he does provide, some what grudgingly.
He takes great pride in his work, and strives for perfection. He hates people who try and belittle what he does. For example saying he can't have a life if all he does is invent tricks. There is always a feeling that Jonathan is under-valued.
His relationships have helped him grow slightly, and he's a bit more confident about his abilities, and his relationships with the opposite sex.