Cynric, as of Dead Ice, is the only pure-blooded blue weretiger, Anita has met outside of the ancient Harlequin animals to call. Blue tigers are so rare that Marmee Noir, herself a master of cats and capable of sensing energies the world round, believes them to be extinct until learning about Cynric. He prefers to be called by the diminutive "Cyn" (pronounced 'sin', leading to the conclusion that the author intends the name Cynric to be pronounced with a soft 'C'), and later changes the spelling to "Sin" because otherwise no one can spell or pronounce it right.
Cynric has short, dark blue hair that looks black in low light, and eyes in two shades of blue with a darker ring around the pupil and a paler ring outside—tiger eyes, but not in a color found in natural tigers. In his animal form he has cobalt blue fur with black stripes.
At 16 Cynric looks very much like a kid and has short hair. In Kiss the Dead he's already 5'9" and still growing, with four of those inches from the last year alone, and has grown his hair long enough to pull it on a ponytail. By the time of Dead Ice he's over six feet tall and handsome.
The White Clan of tigers found Cynric from an orphanage and adopted him. How he came to be there is unknown.
Anita first meets Cynric in Las Vegas where she unintentionally and against her will makes him her blue Tiger to Call while under the control of Vittorio. Cynric's youth makes him particularly vulnerable to the way Anita forges metaphysically bonds with her servants, leaving Cynric besotted with her for an unknown length of time. His level of devotion, however, does not appear to entirely remove his free will.
In Bullet, Cynric comes to St. Louis along with other white weretigers. He remains there and Anita and Jean-Claude plan enrolling him in his last year of high school.
In Hit List, it is confirmed that he is attending high school while maintaining a sexual relationship with Anita.
In Kiss the Dead we see more of Cynric. He is on his senior year of high school, and making impressive performance as a quarterback in new preternatural high school football league, and as a sprinter in running track, likewise at the preternatural league. Given the low number of participants this far, the St. Louis team represents the whole Missouri, and Cynric is a significant reason why the state is doing so well in football that year. A college-level preternatural league started the previous year (or there are currently talks of starting one, see inconsistencies) as well, and college coaches are already scouting Cynric as a future player.
Cynric lives with Anita most of the time, even if he has his own room at the Circus of the Damned as well, and is slowly finding his place among the social structure of her lovers. He isn't entirely satisfied with Anita trying to keep him at an arm's length and wants her to treat him like her lover instead of a kid, which is why he tells her to stop coming to parent-teacher nights if they make her think of him like a child, and part of the reason why Nathaniel is teaching Cynric in secret at the Guilty Pleasures to how to give Anita a private dance. Anita, on the other hand, is angsting over the fact that she doesn't love Cynric like she thinks she should, and his age is still a big issue for her. She still hates his nickname and tries to keep calling him Cynric instead of Sin, and he keeps correcting her.
During the course of the novel Cynric also realizes for the first time how dangerous Anita's job can be. The first awake call is a vampire-related shooting in the news that reports officers down, and shows Anita standing surrounded by bodies. Later Cynric faces the danger more personally when a vampire-wannabe working for a vampire terrorist group bent on releasing vampires from the blood oath structure comes to the Guilty Pleasures wearing a bomb vest and takes Nathaniel, Cynric, Dev, and Nicky hostage. They all assist in their own rescue, piling on the bomber at Anita's mark and holding the dead man's switch pressed until Anita can get close enough to shoot him and the bomb techs get the bomb disarmed.
In Dead Ice Cynric is still in high school and plays football and runs track, although his graduation is close enough that Jean-Claude has given him a brand-new Corvette Stingray in deep, rich blue as an early graduation present. Anita finally realizes her real issue with Cynric isn't his age but that he reminds her of the time when Vittorio raped them both by forcing them into the orgy that tied them together. She is determined to work on the issue, and is even reluctantly considering going to the senior awards ceremony at Cynric's school as his girlfriend, but is unable to find time for it in the end. Cynric fits well within their poly group and seems content in his place in Anita's life, although even after her revelation Anita is reluctant to consider him as the tiger to include in their commitment ceremony, and Jean-Claude sees Cynric more as a beloved nephew and doesn't really want to marry him either, even if the ceremony isn't legally binding.