James "Jem" Carstairs (born 1861) also known as Brother Zachariah is the son of Jonah Carstairs and Ke Wen Yu, a love interest and later second husband of Tessa Gray, one of the protagonists of Cassandra Clare's book series The Infernal Devices and a supporting character in it's sequel series The Mortal Instruments.
Jem was born in Shanghai in 1861 to Shadowhunters Jonah Carstairs and Ke Wen Yu and was raised in The Shanghai Institute. However when Jem was twelve, a demon named Yanluo broke into the Institue and killed Jonah and Ke in revenge for Ke destroying it's nest of offspring and also tortured Jem by feeding him a demon drug called yin fen. By the time the Shaghai Enclave arrived, Yanluo had escaped and Jem had become addicted to the drug. Due to this, Jem still had to take the drug to continue to survive but it was slowly killing him.
Jem was eventually sent to his father's homeland of England to live at The London Institute where he met a boy the same age as him named Will Herondale who at first did not wish to befriend Jem (as Will believed that he was cursed by a demon to bring death to those who loved or cared for him and did not want this to happen to Jem). However after learning about Jem's "illness", Will befriended him (as he believed that since Jem was dying anyway, the curse wouldn't affect him) and they became parabatai at fourteen.
In 1878, Jem (now 17) met a 16-year-old half Demon/half-Shadowhunter Warlock named Tessa Gray and befriended her. The two were briefly engaged, but it broke off when Jem became a Silent Brother (as it was the only way to stop him dying) and took the name of Brother Zachariah whilst Tessa eventually married Will. Despite breaking off their engagement, Zachariah and Tessa would met up with each other on Blackfriars Bridge (Jem's favourite place in London) once a year.
In 2007, after 129 years of being a Silent Brother, Zachariah was finally cured when he came into contact with the heavenly fire inside Will's descendant Jace Lightwood. Soon after becoming mortal again and returning to his original name, Jem rekindled his relationship with Tessa and the two eventually married at Blackfriars Bridge in 2009 whilst being watched by Will's ghost.