Well, Jude's had a very unusual experience because she was living with a guy called Rich, who, in the pilot , turned out to be a demon. She'd had a child with him, so the first episode of the series revolves around the question as to whether or not the child has inherited any of these demon attributes.
That's what originally brings her into contact with John Strange, but after that things progress so they know each other a little more and the investigations proceed with the two of them.