The separation of the Hulk and Banner varies in Marvel comics. Sometimes, external forces or internal conflicts cause them to split, but it's not a consistent occurrence throughout all comics.
No. Marvel Comics is a family - friendly entertainment medium for the most part and does not include such inappropriate content related to sexual stories about its characters like Hulk.
Yes, there are some hints of a special relationship between them. In the movies and comics, they share a unique bond. Hulk seems to have a certain level of protectiveness towards Black Widow, and she is one of the few who can somewhat reach out to the Hulk within Bruce Banner.
Well, start with the basic shape of the head. Make it big and strong-looking. Then, focus on the eyes - they should be intense and angry. Outline the features boldly to give that powerful look.
The main character is obviously the Hulk. And of course, Bruce Banner, who is the Hulk's alter ego. There may also be characters like Betty Ross, who has a complex relationship with both Bruce and the Hulk.