Is there a word for the second part of a story title after a main character?

The word "noun phrase" is a word for all of second parts of the items you list, but I suspect you may be looking for something more specific.

The first element of each example is a noun ("Fitz") or noun phrase {"Harry Potter") and a second noun phrase ("the Tantrums"), or ("the goblet of fire"), connected with a conjunction.

You're right that this title formula is really common! But after searching for quite a while I haven't been able to define a definitive name for the formula or a term for the second phrase.

Some names for the formula as a whole:

• The X and the Y
• The This and the That
• X and the Ys
• A, B and other Cs
• Couplet