[susan53]

Both are acceptable. We talk about something being "in a book/newspaper " etc so that's OK. Here's an authentic example from a concordancer :
isn't there something absurd about a HERO IN a novel who is defeated by his infantile neurosis?

But "of", with the meaning "attached to/belonging to is clearly OK too. Here are some concordancer examples :
For the HERO OF this work by John Osborne and Anthony Creighton is a ...
'How much is Ian Fleming, the author, like the HERO OF his books, James Bond?'
...which would throw him into headlines all over America as the HERO OF a great murder trial.
"The HERO OF his next poem is Napoleon Bonaparte",

There were noticeably more examples using "of" than "in", so you can tell your student that "of" is more frequently used. But both are possible.