FORTINBRA
S: He’s here (or in Norway) because of what an armoured heroine might need.
TOM: He’s three quarters of a particle.
??
R????: Confused lad with a messy barnet, he is – crazy!
B
ILL: Invoice him.
FRO
DO: Even when a bit muddled, he knows where his towel is.
ECTHELION: Formerly supple, before he knotted up.
VO
RONWE: Woven or spun, he stands constant.
THEME: NAMES GIVEN TO MORE THAN ONE CHARACTER.
ONCE LITHE = ECTHELION, indeed.
FORTINBRAS is correct, and you are right that I primarily think of him as the third avenger of 'Hamlet,' whose words finish the play, and perhaps momentarily forgot that he's two hobbits as well, rather than a steward or something.
However, it's not 'fought in bras.' Which is not a grammatically correct answer to the clue.
And hardly does full justice to the 'armour' element. And I would have added something like 'sounds like' if the spelling was wrong, wouldn't I?
'Because' gives you FOR.
'Armoured heroine' suggests TIN BRAS.
Which an *armoured* heroine might need.
And that is why the letters appear ('why he's here').
(I added the Norway reference because he's not a well-known Thain and I thought it might be a bit difficult without it).
Still, it's rather wonderful that 'fought in bras' is a homophone of 'for tin bras,' which I wouldn't have known if you HAD got the elements right.
You still have the barnet one to guess, don't forget! Chop chop!