What would you write when referring to a fictional character in a non-fiction work? e.g. Tom Sawyer is a boy, he has no parents, he goes on adventures or Tom Sawyer was a boy, he had no parents, he went on adventures.
If the non-fiction work is an essay or similar analysis of the literary work, I would use present tense.
Tom Sawyer's friendship with Huckleberry Finn represents unity between middle and lower class Americans. In the scene where Tom is painting his fence...
(I've never read the book, I just made something up for an example)
The book exists in the present, so the characters do as well.
The author, though, exists in the past, since he's no longer with us.
Mark Twain wrote about a boy, Tom Sawyer, who has adventures with his friends.
It would not be wrong to write about the character in past tense if describing his actions that have already happened (because you finished the book). But present tense is okay here too.
Tom Sawyer nearly spoiled his friend's plan to run away.
Tom Sawyer nearly spoils his friend's plan to run away.
A rule-of-thumb is that characters may become part of the past only in their universe, where they are a "real" person. In this case, you can use the past-tense if you are referring to a "previous version", or "younger version" of the character. You would still use the present for the contemporaneous one.
In any other universe, e.g. in those where they are fiction, they are elevated to timeless absolutes. The use of the present-tense is preferred. This should hold true for most non-fiction works about fiction.