Jeremiah “Harbinger of Doom” Avery (ex-Jason) is a human character from the “real world”. He was briefly introduced in the Second Chronicles as a victim of Lord Foul’s summoning ritual, and later gets adopted by Linden (poor guy). By the time of The Third Chronicles of Thomas Covenant, Jeremiah has become a central character and becomes one of the three point-of-view narrators in The Last Dark.
During the span of the 2nd and 3rd Chronicles, Jeremiah grows from a small child into an adolescent boy. His garystueish, color-shifting eyes have a hue described as “loamy” (which is author-speak for "shitty brown"), his hair is dark and a fuzzy stubble develops on his cheeks while separated from his mommy for a couple of days. Linden clothes him in horse-patterned blue pajamas, which may just possibly bear mystical meanings akin to her grass-stained spiritual-map jeans, but actually is far more likely to show what a deranged and cloyingly over-zealous mother Linden is, amongst her numerous other irritating faults.
Jeremiah has a maimed right hand, much like Covenant's. Jerry's disfigurement however was caused by his birth-mother - who quite staggeringly was even more dribblingly crazy than Linden... how unlucky can one guy be, fercrissakes? - forcing his infant hand into the flames of a bonfire used to conjure up Lord Foul.
In the Third Chronicles, Jeremiah is initially presented to the reader as an entirely unresponsive, incapable of any communication and lost inside his own head. There are several theories as to why he has entered this nigh-on catatonic state:
i) As a result of the trauma of his maiming as an infant
ii) As an act of rebellion against his adoptive mother for sickeningly forcing him always to wear cutesy horsey pajamas, when all the other kids in the neighbourhood are dressed in Abercrombie & Fitch™ and SuperDry™.
iii) As an act of disgust against his adoptive mother for not letting him paint his bedroom walls black, not letting him get a tattoo and confiscating all his Death Metal CDs. (Protip: Acquire a Finnish surrogate parent. In a country where infants get breastfed to the tempo of Hevisaurus, the sudden absence of one's metal CD's is likely explainable by mommy and daddy having set up an ersatz moshpit in the living room.)
iv) As an act of protest against invariably being given such age-inappropriate Christmas and birthday presents. I mean, who the Hell would want a toy racecar or a box of wooden building blocks, even at his age? For the love of God, get the kid an Xbox™ or an iPad™, Linden you skinflint.
v) As a purely instinctual survival mechanism to remove himself entirely from the brain-curdling bouts of Linden's constant whining and blubbing.
vi) As a result of an over-indulgence in brightly-coloured shrooms that he found in Linden's garden.
Serious Donaldson scholars utterly discount the first theory, but argument rages over the likelihood of all of the following five.
Early on in the Third Chronicles, Linden is briefly fooled into thinking that Jeremiah has recovered from his ur-autism, but it turns out that he was merely possessed by a croyel. How we readers laughed at the crushed look on Linden's face, once this was finally revealed.
While trapped within the dungeons of his mind, Jeremiah's expressions could be best depicted as ranging from “absent” to “drooly”, but lo and behold, what a hippity-hoppity zestfest follows, when he finally awakens...
Once back to reality - if it can be called such - Jeremiah maintains that, during his long mental absence, he visited the Land many times via a kind of astral projection. Most discount this embarrassing claim as the lingering remnants of his being off his face on shrooms.
Speaking of embarrassing, Jeremiah has reached that awkward age, halfway through puberty. He gets very irritated at not being able to understand some of the Swordmainnir's bawdier jests. (He did very briefly consider asking his mother about these, but the hideous thought of Linden sitting him down and having "the talk" with him threatened to tip him over into catatonia again, so shuddering, he abandoned the idea completely. Then again, some education about flowers and bees would be highly advisable to avoid further discomfiture. You do not tell a busty Giantess in her prime that "she probably has never moaned in her entire life" without a hearty sniggerfest among the "big girls".)
Special genius cells inhabit Jeremiah’s brain. A wizard of constructs, he is able to build almost anything when presented with the right materials, possibly even a new Arch of Time out of breadcrumbs and Rainbow Dash’s tail. If only he'd not been swept up and conveyed to the Land, he could have signed up to a major sponsorship deal with Lego™.
So far Jerry's been seen to build himself a rehab centre out of quellvisk bones (which allowed him to snap out of his catatonia) and a sanctuary out of malachite for the Elohim to protect them from being gobbled up by The Worm of the World’s End. This latter at least was indeed A Good Thing™, because it meant that the life of Infelice, the Elohim queen, was saved, allowing her to be drooled over on an ongoing basis by any male being with a pulse. (Damn, but she's a hottie).
Following the dynamic trio's final victory at the end of The Last Dark, things look worrying for Jeremiah, in that he, Linden and Covenant may have become immortal through their arcane exertions in saving the world. This is a fate too horrible to contemplate for poor Jerry - he'll forever be stuck as a sulky pubescent teenager with raging hormones and equally raging acne, being dragged along unwillingly forever in the wake of his mother and her gaunt boyfriend, making sickened gagging noises as the latter play kissy-face for all eternity. I'd bet that Jerry would rather that the world had come to an end after all...