Grief. Sadness. Longing.
Brother Zachariah knew that Silent Brothers lost their emotions when they became a part of the brotherhood, and yet he still felt himself longing for the good old days, with Charlotte, Henry, Will, Jessamine, and Tessa. He longed for the days when people still called him Jem.
Jem's life had been stuck in black and white since his introduction to the yin fen, completely devoid of colour. Jem remembered how his parabatai Will had come close to colour-a faint, almost golden glow lighting him from within-but the glow was not quite enough to actually become colour.
Jem knew he was dying. He only wished to see colour once again before he went. And somehow, miraculously, his wish came true. He was playing his violin one night, letting out the sorrow that was consuming him, when his bedroom door opened behind him. He had assumed it was Will, since everyone else in the Institute would knock first, but he had assumed wrong. Slowly, he turned around to face what could only be described as a girl who was as radiant as the angels themselves. She was breathtakingly beautiful. And Jem could see her in colour.
Her name is Theresa Gray, but everyone calls her Tessa. She was shy, a book-lover, and enormously kind. Jem knew she was the one as soon as he saw her. She had brought back the colours. Now, there is colour everywhere he goes, and the world is beautiful. Tessa with her lovely brown hair, and her storm-gray eyes that are windows to her soul. Will with his jet-black hair, and his impish sky-blue eyes. The only problem with the world is that everything must be balanced; the angels with the demons, beauty with ugliness. And Jem believed he was ugly. His hair and eyes the colour of the cursed yin fen, his body so skinny that you could count all his ribs, his paper mache skin, scarred from all the runes that had been drawn on him. The only two people who were completely blind to his ugliness were Tessa and Will, and he knew he could not live without them. Will was already his parabatai; he proposed to Tessa soon after.
As it turns out, Jem and Tessa never got married because of all that happened with Mortmain. Tessa had been kidnapped, and Jem was on his deathbed, the world slowly sinking back into black and white. The only acceptable way to keep Jem from dying was to make him a Silent Brother, and he agreed with regret, knowing that now he would never get to marry the girl who brought back colour.
Soon after becoming a Silent Brother, Jem was renamed Brother Zachariah, and all of the colours in the world had disappeared. Back to black and white.
Years passed, and Brother Zachariah watched Tessa marry Will, watched them have children, and watched Will grow old. He was there when Will died, let Will hear him play one more song on the violin before they parted forever. Brother Zachariah didn't feel anything that day. He didn't feel sad, or grieved, because he was a Silent Brother. He had no emotions.
It was just over a century later when Brother Zachariah felt something again. There was a war going on, and many of the Silent Brothers had already been killed. Brother Zachariah knew that he could be next, so he took a moment to remember the past, everything that made him who he is. He remembered how his life was in black and white, then how Tessa had brought back colour. He remembered Tessa and Will's wedding, and everything that happened after. How the world had sunk back into the colourless void. And for the first time in a long time, Brother Zachariah felt emotion. He grieved. He longed for the days when he had both Tessa and Will by his side. He wished he had been the one to marry Tessa, grow old with her. He had loved her. And he realized, he loved her still.
Weeks later, Brother Zachariah was fighting among Jace Herondale, Clary Fairchild, and the Lightwood siblings when Jace was injured. Brother Zachariah attempted to heal Jace, but the heavenly fire within Jace got out of control, and consumed the Silent Brother. He fell to the ground, burning, wishing that he could have seen Tessa one more time before he died.
Brother Zachariah woke up a few days later, noticing that something was different, but he wasn't quite sure what. He blinked a few times, and sat up. He noticed that he was in a small bedroom, but he didn't know where he was. Alicante? New York, perhaps? All the questions in his head faded away when someone opened the door. "You're not a Silent Brother anymore, Jem," he heard the person in the door say. He looked up, and all the colours came rushing back. Tessa had come back.