Is 'the heroine' also a translation for hjälten?
It is now.
Thanks. 'Hjältinnan' would also work?
Yeah, though I don't think I'd say it myself, it sounds a bit old-fashioned to me.
I'm guessing indefinite is "Saga", could saga have the same definition in English or is it reserved for fairytale?
Yup, indefinite is saga. Its meaning is colloquially restricted to fairytale, but like in English, it can be used for old chronicles too (e.g. Njáls saga).
Shouldn't "protagonist" be accepted as well? I believe this is the correct translation, nobody calls the main character "hero" in English.
In formal discussion of literature maybe nobody calls the lead character the 'hero', but it's not unusual at least in some dialects when talking about stories specifically written for children, especially when discussing them with children.