I think Barrons is Draghar... not only Draghar Druid trained, but all or part Draghar.
Just as KMM used Norse legend for Berserkers, I think she again was inspired by Norse in creating her Draghar, using both of these as sources:
1. Draugr: Old Norse for "dragon", a winged serpent.
2. Draugr (plural Draugar: Undead creature in Norse mythology.
a. Literally means "spirit who guards the burial mound (barrow) of a king", jealously guards treasures; the body, not just soul, of the dead;, does not decompose/corrupt; contains alter ego of deceased, his representative, retains his character, i.e. if good/kind in life, remains so as draugr in pre-Christian belief (from Viktor Rydberg's "Teutonic Mythology" p.742) *
b. Able to leave dwelling place (barrow) and visit the living during the night, often ending in death of one or more living creatures.
c. Can increase their size at will, size being an expression of its vast strength.
d. Immune to usual weapons, can only be defeated by Hero in hand-to-hand combat; almost impossible to kill, may come back (likens it to vampires in legend).
e. Move magically through earth, swims through solid stone.
f. At times, exhibit powers of a magical nature, possess knowledge of the future, can control weather, shape-shift.
g. Believed to long for the things of life, envy those yet alive.
* Rydberg link too long. Google "draugr thrainn"; link starts with http://nsl-archiv.com/Buecher... ; enter "draugr" in in-site search
All traits listed above tie to Barrons in some way:
1. Serpent/snake references to JB by Mac, winged serpent tattoo on Draghar Druids/followers (DH), Druids using dark magic.
2a. JB guards treasures (relics) and treasures his belongings (Persian rug).
b. Usually out at night, often returns with blood on him.
c. Mac frequently mentions his taking up more space than he should.
d. JB's speeches re killing man who broke him "with my bare hands", and about difficulty of killing vampires, other creatures "impossible to kill" (DF).
e. At O'Bannion's vault door (DF) JB's "I'm a good swimmer" could hint at swimming through walls, not only water; says he could "continue swimmingly" without her; he defies laws of physics.
f. JB's "I know everything" statement implies he's training/preparing Mac for future he knows she must face; fog is weather control; his sudden appearances defy laws of physics.
g. JB's reaction to birthday cake, his reactions to blood, interest in Mac's neck (though that could be plain lust) all could indicate envy of the living.
February's Teaser, "I was mighty.. They didn't recognize me.. I would remind them": Draghar were destroyed, some Druids scatterred, but Fae Court may not know Draghar had learned secrets of draugr-immortality. Or this scene took place long, long ago and JB was not recognized as threat (his powers not stolen). He could have been one of the Druids who were scattered and turned Draghar/draugr later; or he was rescued by 'V'lane's Princess'; or JB is Draghar/human hybrid.
JB's curse may be his immortality (degree of undead?). I agree with whoever suggested JB could be long-dead Keltar, either forgotten or stricken from family records as Cian was -- another reason to feel cursed -- all Draghar were considered 'damned'. Several phrases from Keltar couples reappear between JB+Mac, and it would make a tidy wrap-up to the combined series.
So, putting all my eggs in this basket, my theory is:
1. Barrons is Draghar.
a. Exact connection to draugr (dragon/undead) unknown yet, but exists.
b. Draghar gained immortality, with unfortunate side-effects of undeadness.
2. Barrons is guarding the barrow of royalty (King or Queen) and royal treasures (Fae relics).
a. BBB is the bastion(projecting part of a fortification or stronghold) over the barrow and vault.
3. Barrons, one of "the ancient ones", has returned to reclaim what TDD had stolen from them -- maybe powers, but also Druids' good name, right to exist.
a. With Fae rulers incapacitated, balance of power has shifted, so "might makes right" is truer than ever. Long ago, Fae had the 'might' -- now, maybe not. This also explains the enmity between JB and V'lane: Draghar vs. Fae.
b. Mac is at fulcrum of at least 3-way power struggle. Mac is epic, a powerful weapon.
c. Source and extent of her abilities is unknown to her and us, not so unknown to JB and V'lane. (Known to LM? Rowena?) No wonder they all want to harness her powers to increase their faction's 'might'.
4. Barrons kills when necessary, out of mercy and with honor.
5. Barrons says he will never let Mac die. If she dies or is too near death to recover, JB will turn her Draghar/draugr as he is -- maybe not the HEA the way we wanted, but a HEA for Mac+Barrons just the same.
6. Draghar/draugr are related and superior to Hunters; were betrayed by Hunters when Hunters sided (as mercenaries) with TDD against Draghar long ago.
I could be wrong. But until theory disproved, I see definite connections between Barrons/Draghar/draugr. (Also strong connection between Barrons/Keltars, but that ventures into 'who', not the 'what' addressed here.)
Comments welcome. (I have chocolate, so am prepared for anything.)