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This article contains plot details for the newly released A Crystal Of Time. You have been warned.
Japeth is the Snake's true name. The "Snake" is revealed to be the twin of Rhian, borne to Evelyn Sader and King Arthur through means of black sorcery.
Japeth supports his brother Rhian to become king at first because he and his brother made a deal: Rhian would become king and, in return, he would bring back Japeth's one true love from the dead. It is assumed by both the reader and Rhian that this true love is their mother, Evelyn Sader, but later revealed that it is Aric, Rhian's most hated enemy. When Japeth reveals this to Rhian, Rhian refuses to, switching the roles of the two brothers from their prophecy.
Rhian and Japeth both received the same prophecy when they were younger, that one of them would go on to become the one true king, who would marry his love, while the other brother would be healed by her blood. This holds true throughout most of A Crystal of Time, but when Rhian breaks his vow to bring back Aric, their roles are reversed. Since Japeth would no longer be willing to help Rhian become king, their roles are reversed, making Rhian the brother who is healed by blood and Japeth the one true king. This happens right before Rhian and Japeth have a huge fight, which results in Rhian killed at the hands of his own brother, impaled upon his "own" sword, Excalibur.
A New Identity
After ruthlessly slaying his brother and cackling over his dead body, Japeth takes on the form of his brother Rhian and re-announces his wedding to Sophie. Sophie is at the mercy of Japeth's scims and can't do anything about it.
There are no weaknesses revealed about Japeth except his love for Aric, which hasn't really been proved a "weakness"
Japeth has no interest in women, so he is not going to fall for Sophie's charms the way Rhian did in the end. Nor does Japeth have any sympathy for Good or Evil.
Japeth is identical to his brother Rhian in every way except his scims and his paler skin. Both brothers have copper hair, blue eyes, and a strong body. However, Japeth is unlike his brother in that he is Evil, so his features are often described as colder. When put out to the sun, he is described as "looking like a burnt ham".