1. Orphan (the character in the beginning, with their life lacking something)
2. Wanderer (the character after the initial shove into the main problem, figuring things out but still half likely to reject the whole quest)
3. Warrior (the character taking on the quest and finding new courage as well as dangers)
4. Martyr (the character being willing to put everything on the line to achieve the goal, though they don't actually have to die [but can if this is a tragedy])
This is based on Carol S. Pearson's The Hero Within, and has been adapted by lots of writers. This post claims that these four character stages can be overlaid onto the three-act plot structure thus:
In Act 1: ORPHAN...lost...needing a quest.
In Act 2A: WANDERER...chose the quest...but unsure how to achieve it.
(stick in here a MOMENT OF GRACE (MOG) where S/He discovers what the story is really about)
In Act 2B: WARRIOR...having discovered through the moral premise how to actually get what he needs.
In Act 3: MARTYR for what he and his village back home needs.
This lines up a lot better for me, for the stories I write. Whew!
As for Frodo...hmm, does he become a Warrior at "I will take the Ring to Mordor," or not until the breaking of the Fellowship? Still undecided for me, but maybe the latter. It's only then that he realizes (moment of grace) that he can't do this with the whole Fellowship along, but must do it anyway. Plus then the LOTR trilogy breaks more nicely into the three acts too. :)