In Eduardo's case, Nina created him in a moment of fear so I doubt she had time to dream up every last detail about him. She needed someone fierce, or at least fierce-looking, and there he was, so I think the actual creative process is at least partially instinctive. Each Imaginary Friend might be created partially from instinct, partially from deliberate design, the proportion of each depending on the creator and the circumstances under which the IF is created, with both parts constrained by whatever limits the creative process has.
Good questions, by the way.
