Jonas’ or Jonas’s or ….?

I wrote a fictional story for English class and the main character is called Jonas. In my fictional story, I don’t know if I use Jonas’ or Jonas’s?

You can make a case for either. Just pick one and stick to it. I prefer Jonas’, which probably means Jonas’s is the accepted way of writing it by now.

Jonas’

Living languages change. Most publishers now prefer the apostrophe-s possessive regardless of whether the name or word ends in an S or a double-S. So while you won’t look foolish submitting work with Jonas’ [noun], an editor is going to change it to Jonas’s [noun], the boss’s face, Jess’s hands, etc.
FWIW, Jesus the religious figure is still Jesus’. Hispanic people named Jesus still get Jesus’s.

Jonas’.
That said I avoid characters with final “s”‘s in their names just because of that.

Jonas’

Answer 6

Both ways are correct. Jonas is no different that John. They can both have an apostrophe s. You can also do s apostrophe.