Kit and Ty shared a close friendship up until the end of Queen of Air and Darkness and were frequently seen together. They are the "Sherlock" and "Watson" of the Shadowhunters fandom, often embarking on quests together and usually never seen without the other. Understanding autism unlike other Shadowhunters, Kit is able to connect with Ty on a whole other level, being able to calm him down when no one else can. This resulted in them sharing a very close bond.
Due to Ty's point of view not being written in the books, it is unknown what his true feelings for Kit are. Kit is shown to worry about Ty's well-being frequently, and often thinks of him as "beautiful" or "elegant". Other characters have thought of them as close, noting that they are good friends and that Kit is always hanging around Ty, and vice versa. (Dru even noted that Kit "meant so much to Ty" at one point, due to her being worried since Kit was wounded.)
Ty was also one of the reasons that Kit decided to become a Shadowhunter and remain at the Los Angeles Institute.
Ty had snuck out of the house to follow Julian and Emma while they were questioning Johnny Rook and ended up breaking into the basement where Kit was. Thinking Kit might attack Julian and Emma, Ty attacked him first, holding a knife to his throat and demanding answers. Kit was struck by how beautiful Ty was, especially his eyes. Julian and Emma, hearing the noise found the two and all three Shadowhunters were forced to leave.
Later after his house was attacked by demons and his father was killed, Kit was brought to the LA institute where it was revealed that he was a Shadowhunter, specifically a Herondale. Kit locked himself in his room at the Institute, not really accepting his Shadowhunter lineage. Ty waited outside the room, saying he was protecting him, in case anything happened.
Lord of Shadows
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During Lord of Shadows, Kit and Ty grew remarkably closer and became good friends, starting from the moment that Ty found Kit in the kitchen sneaking cookies. Along with Livia, Ty's twin sister, the trio often spent time together, and would do things like spy on the Centurions. Kit and Ty also shared many moments, perhaps most notably the following:
- At one point, when Livvy was hurt after a dangerous trip to the Shadow Market, Ty had a panic attack and went to the roof of the London Institute with Kit following him. Kit asked him what he needed ("Tell me what you need"), and Ty asked him to hold him ("Put your arms around me. Hold on to me"). Not knowing what else to do, Kit did as Ty wanted. Neither of them pulled away, likely for a long time.
- When discussing whether or not Ty would go to the Scholomance, Ty said, "Livvy would miss me -" Kit then interjected, saying "Your whole family would miss you. And I would miss you." After hearing those words, Ty blushed.
- Ty was the one who gave Kit his first rune, and later the Voyance rune commonly seen on Shadowhunters. Interpreting something the wrong way, Ty tried to give Kit the Voyance rune before being refused. Upon seeing Ty's disappointment and humiliation, Kit relented.
Throughout the book, Kit made continuous references to Ty's behavior, noting that most people didn't understand him, feeling protective of him, and describing his appearance in (as noted by fans of the ship) oddly poetic ways. This, along with the many reblogs of Kit/Ty posts on Tumblr made by Cassandra Clare, practically canonized the ship, and a Kit/Ty confession became something many fans of the series wanted in Queen of Air and Darkness.
Queen of Air and Darkness
Kit and Ty's relationship became much more developed in the last book of the Dark Artifices. At this point, Kit appeared to at least have a crush on Ty, and their relationship was further implied to be romantic due to the many posts about them on Cassandra Clare's official Tumblr and the fact that neither of them were straight, as Clare said in an interview.
Kit was shown to be incredibly worried about Ty after Livvy's death and sat with him while the other slept, not leaving his side. When Ty proposed a plan to raise Livvy from the dead, Kit was uneasy to do so but agreed due to worrying that Ty would shatter completely if he didn't. Their story in QoAaD mainly followed these plans and Kit's growing feelings for Ty.
There were many hints in the book which further canonized Kit's feelings (such as his heart turning over when he looked at Ty which was pegged as a familiar feeling for him, or saying things that he liked about him) as well as signs that Ty may have romantic feelings for Kit, relaxing when he saw him, tucking his hair out of the way and voluntarily hugging him at one point. Ty became slightly distant to all others, but trusted Kit without hesitation.
When they finally began the process of resurrecting Livvy, Kit quickly realized that it was wrong and begged Ty to stop, officially declaring his feelings for him ("I love you, Ty. I love you."), as well as saying that they "will go through it together". Ty was shocked by his declaration, but nevertheless proceeded with the ritual - however, due to having a corrupted catalyst, Livvy was not fully brought back and returned instead as a ghost who seemed to barely remember Kit (referring to him as "Herondale" in a very aloof manner). Ty told Livvy that there was nothing without her, and broke down crying when she vanished - however, Kit, who usually would have attempted to comfort him, was so stunned and hurt by what Ty had said that he did nothing.
After the ritual, Kit became much more distant but returned Livvy's locket (which they had used for the ritual) to Ty, who began saying something ("I thought that it would be -"). This could have led to a discussion about Kit's confession, but the two were interrupted.
Sometime during the battle, Kit and Ty were chained to a tree by Magnus in order to prevent them from joining and fighting as a safety measure. Kit revealed that he was angry at Ty, which prompted the other to say angrily that he thought Kit cared but he really didn't. This made the situation worse due to Kit being humiliated still from his confession, leading to him saying that he wished he'd never known Ty. Ty cried, though it is unknown whether this is from him being hurt by Kit's remark or because Livvy's resurrection was not successful. They were interrupted again by Livvy's ghost, who freed both of them. Not long after, Kit was dragged off by Emma. This was the last time he and Ty were seen together in the series. After Kit made the Riders' steeds disappear due to magic from his faerie blood, he became under the impression he was dying and thought, "Tell Ty -" as his "last thoughts" before he passed out and was taken to the Silent City.
He then agreed to move to Devon with Tessa and Jem when Tessa asked, making an excuse that it was because he didn't want to put the Blackthorns in danger when really he was thinking it would not make any sense for him to stay after he'd spent all his time with Ty (due to him supposedly not meaning anything to Ty), since he didn't know the rest of the Blackthorns save Dru very well and knew he would feel like an idiot seeing Ty every day. Ty, at one point, was shown to be wondering where Kit was and asking to say goodbye to him, before Magnus explained to him the severity of what he'd done in resurrecting Livvy.
Before leaving, Kit saw Ty on the beach as a sort of final goodbye requested by him, deciding to remember him happy and smiling. Jem and Tessa seemed deeply sympathetic, and it was said that Tessa looked at Kit as if she knew more than what he'd told her.
Kit and Ty (along with Dru) are going to be the main protagonists of The Wicked Powers, which will be published sometime in 2022. According to Clare, as of now it will be the last of the Shadowhunter books, chronologically and likely in publication. The Wicked Powers will likely explore Kit's heritage, the consequences of the ritual the two of them performed, and their relationship.
They will also be featured in the final two stories from the collection "Ghosts of the Shadow Market", "Ghosts of Old Loves" and "Forever Fallen". "Ghosts of Old Loves" is part of a Sherlock Holmes quote ("of all ghosts the ghosts of our old loves are the worst") so it is possible this story will address their relationship.
Shipping between the two began immediately after the release of Lady Midnight, despite there being very few interactions between the two. They became the dominant pairing in the fandom. Fans were hoping for more interaction between the two in future novels.
On AO3 it is the most written ship for both characters. It is also the most written ship in The Dark Artifices tag. As of late December 2018, they have just over 200 works.
Fanfiction for the two that takes place in-canon is divided into three categories primarily:
- Post Lady Midnight - since Ty and Kit didn't know each other very well yet, the fanfiction from this area focused mostly on their developing relationship (usually short one-shots of them getting to know each other).
- Post Lord of Shadows - by this point, fic for the two became much darker. Due to Ty's reaction after witnessing Livvy's death, it focused mostly on Kit comforting Ty.
- Post Queen of Air and Darkness - due to the depressing ending the two characters got in QoAaD, fanfic for the two will, from now on, likely include reunions or post-canon fics. In these fanfictions, Kit is usually shown to be pining after Ty until they sort everything out, finding out that their feelings for each other are actually requited. Kit's pining causes some fans to jokingly say that he is a true Herondale due to the angsty aspect of relationships that Herondales often end up in (Jace thinking he and Clary were related, Will thinking he couldn't love anyone because he was cursed).
- It is confirmed that Ty is "definitely" taller than Kit, not only making Kit the first major Herondale to be bisexual but also the first major male Herondale to be shorter than his love interest.
- The two of them share some parallels with another couple from the upcoming series The Last Hours, James Herondale and Cordelia Carstairs. For example, like James, Ty does not look like the rest of his family (James has gold eyes, while Ty has black hair and gray eyes unlike the rest of his siblings). Like Cordelia (according to the Trivia section of her page on the Shadowhunters wiki), Kit was raised with not much experience with friends and next to none with romance, and traveled often. Cordelia is also said to be stubborn, brave, and smart, which fits Kit's personality as well.
-Ty also shares similarities in character development with James. Like James, Ty is quiet and studious and likes to read, and grows up in a very loving environment. However, just like James was greatly affected after Grace broke his heart, Ty was greatly affected by Livvy's death. These events caused both of them to become more troublesome and difficult to reason with.