Multiple translations.

Prequels:

Golding, Julia, The Companions Quartet. Book One: Secret of the Sirens, Oxford: Oxford University Press, 2006.

Golding, Julia, The Companions Quartet. Book Two: The Gorgon's Gaze, Oxford: Oxford University Press, 2006.

Golding, Julia, The Companions Quartet. Book Three: The Mines of the Minotaur, Oxford: Oxford University Press, 2007.

Spin-off:

Golding, Julia, Universal Companions. Book 1. Water-thief. (E-book). (S. l.): Frost Wolf, 2011.

In the final volume of The Companions Quartet, the threat to Connie, the universal companion, from the evil shapeshifter, Kullervo, continues. This time, he is using a permanently conflicted mythological creature, Chimera, a potential companion to Connie's younger brother, Simon, who now joins her and her aunt Evelyn but dislikes anything connected to the Society for Protection of Mythological Creatures. Chimera is an evil, three-part (snake-goat-lion) hybrid violently hostile to humans. The trustees of the Society are able to determine that Simon is companion to two different categories, Sea Creatures & Reptiles and Four-legged Beasts & Beings; and specifically to three species of creatures: the Almathean goats, Nemean lions, and great snakes.

After the first encounter with Chimera when Connie and her friends barely escape death, she is forbidden from entering the moor where the creature is lurking. Still, she feels the growing menace and presence of Kullervo, and despite the trustees' prohibition, she is convinced that she must confront him and attempt to vanquish him once and for all. She learns that previous universals wanted to do that and failed; they died or became his captives. She tries to find out all she can about these attempts but is refused access to the Universals' library through the intrigues of Mr. Coddrington. Left to her own devices, she trains with various protective and aggressive mental tools available to a universal and is finally able to use them promptly and efficiently. 

In the meantime, Kullervo is massing troops of his supporters on the moor. Connie's cousin and best friend, Col, companion to the Pegasi, is lured there when riding his horse Mags and stunned by the deadly stone sprites. Connie alerted to Col's disappearance by their friend, Rat, uses her universal abilities to sense where Col was and in spite of the ban, enters the moor. Once they found and revived Col, they realized that crowds of hostile creatures serving Kullervo have surrounded them. Kelpies taking the shape of horses were pretending to graze close to Mags, and led the whole group right into a bog. Once both boys are sinking into the mud, the Chimera attacks. Connie is able to frustrate the hybrid's assault using her shield and then her longbow. She then pulls out the boys who by that time had been almost entirely swallowed by the bog. Connie's mental appeals for help go unanswered and the shield against the stone sprites she extended to her two friends could not save them, as the Chimera announces that Kullervo was asking for Connie. Between the stone sprites and the Chimera, there is no escape. Connie promises to let the Chimera take her to Kullervo if her friends are allowed to escape. The hybrid agrees and taking Connie in her jaws she goes to join her master. 

The boys are taken by two dragons swooping in from the sky. The Chimera brings Connie to Kullervo at the site of the oil refinery. There, the shapeshifter blackmails her into collaborating with him, threatening to kill her friends kidnapped by the dragons. She challenges him to a mental duel, providing he first lets her friends go. Kullervo agrees. The struggle in her mind begins, the two companions merge and at first Kullervo forces Connie through many transformations into evil creatures, but she finally transforms her mental energy into her circle of friends. She offers Kullervo the chance to change as an equal into a form that he hates, the human form. He violently refuses. When he does, he is absorbed into her mental energy and his identity as Kullervo ceases to exist. Connie, the universal companion, is now able to transform into any mythological creature she wants, having acquired Kullervo's shapeshifting abilities. 

Once Kullervo is gone, the Society ends the war on the moor and calms down Kullervo’s supporters. Connie's aunt Evelyn gives birth to her baby, a boy whom Connie's special mental skills recognize as another member of the Company of the Universals, the fifth company of the Society.

Each volume begins with a brief definition of the title creature quoted from The Oxford English Dictionary. The quotation highlights the mythological origin of the creature and its standard characteristics replicated in the contemporary being. The Chimera, a hybrid creature, is the main mythological character of this volume. It is not the "original" monster, daughter of Echidna and mother of Sphinx and the Nemean lion (Hesiod, Theogony 256–265, in Barry B. Powell 2017 translation published in Oakland, CA by University of California Press):

"Chimaira breathing deadly fire, terrible, huge, swift-footed, and powerful. She had three heads: one of a savage lion, one of a goat, one of a snake, a mighty serpent. In the front she was a lion, in the back a serpent, in the middle a goat, breathing out the awful strength of blazing fire. Pegasos and noble Bellerophon killed her. Chimaira gave birth to the Sphinx, the bane of the Kadmeians, seduced in love by Orthos; and the Nemean Lion, that Hera, glorious wife of Zeus, raised up and settled in the hills of NEMEA, a plague to men."

Golding's Chimera, like the Sirens, the Minotaur, and the Gorgon in the three preceding volumes, represents a species, not a particular mythological individual; her appearance corresponds to Hesiod's and Homer's (Illiad, 6, 180) descriptions. Book three of the Companions Quartet presents an intriguing explanation of the character of Chimera. The fact that it is composed of three different natures makes her continually conflicted with herself and instead of acting together in harmony, each nature struggles for domination which is fleeting and can never be won for long. Kullervo was able to control Chimera because being a shapeshifter he transformed into another chimera and ended the creature's permanent solitude which was the source of her deep unhappiness and the resulting hate. Kullervo was using Chimera as an obedient servant in his plans to destroy humanity, even in his Chimera form, he did not experience any sense of kinship or compassion. He treated her instrumentally. 

Golding, in her attempt to champion close and mutually satisfying interaction between people and nature, invented the concept of a companion, someone who is attuned to a specific mythical being and provides companionship and friendship. After Kullervo's demise, Simon, the new companion to Chimera, calms her down offering each of her three natures internal peace and rest from violent conflict, she enjoys this new kind of companionship which produced a satisfaction that was drastically different from Kullervo's lust for destruction.

In this volume, Kullervo pronounces a general condemnation of humanity intent on selfish and short-sighted exploitation of other species and nature itself, endangering the world’s future. He also focuses on all of the destructive aspects of human relations with other humans. While he is undoubtedly an enemy, he symbolizes nature fighting back against humanity, as well as the destruction such a fight must bring. 

Golding uses Greek mythology and its creatures to give nature a robust and complex set of personalities, as opposed to treating it as a nameless, non-sentient mass to be exploited for selfish needs without a second thought. The novel deals with many themes highly attractive to teenagers: a lone young saviour with special powers, stereotyped teenage characters such as the collest boy, a snobbish girl from a wealthy family, immigrant kids dealing with cultural differences, unreasonable teachers, parents wrapped up in their work, patronizing adults, older siblings’ responsibility for the youngermembers of the family, early responsibility, etc. All these themes mingle together against the background of an ecological catastrophe and lack of respect for nature.

Smith, Jules, Critical Perspective, London: British Council, 2013. Retrieved on August 26, 2020.

Science Has The Best Stories – Introduction from Julia Golding, 'Best Selling Author', June 13, 2018 (accessed: September 17, 2020).