One of the most complicated relationships in literature, Jace and Clary are each other’s main love interest throughout The Mortal Instruments series.
They first see each other at the Pandemonium Club, when Jace, along with Isabelle and Alec Lightwood, pursue a demon there. From the very first sight, Jace and Clary find each other annoying but intriguing. They find themselves thinking about the other, even while outwardly insulting one another – Clary having even slapped Jace. As they spend more time together, the annoyances begin to disappear and they are drawn to each other, slowly falling in love. They share a passionate kiss on Clary’s birthday. A few misunderstandings here and there, and they are stopped from forming a real relationship, even though both clearly love each other.
Soon afterwards, Valentine Morgenstern, Clary’s father, tricks them into believing that they are siblings, horrifying them. Jace especially visibly becomes distraught.
In the following few weeks, things become awkward for both Jace and Clary because despite finding out they are brother and sister, they cannot rid themselves of their romantic feelings for one another. They try to deal with it by entirely avoiding each other, communicating only when extremely necessary. Clary attempts to start a relationship with her best friend, Simon and Jace also hooks up with Helen Penhallow – both attempts backfiring because of the mounting jealousy for the new person in the two lovers. These weeks are harrowing for both Jace and Clary who are forced to share a passionate kiss at the Faerie court and also almost commit incest in the heat of the moment while in Idris.
When Jace, again mistakenly, finds out he has demon blood, he realizes that maybe this is why he feels what he does for someone who is supposed to be his sister. He gives in to his passion and love and offers to start a clandestine relationship with Clary. The latter refuses on grounds of it being ‘sickening’. Jace is horrified that she would think what he feels for her could be sick, creating quite an irony at the situation.
Later, Jace and Clary share a platonic night together and Jace leaves her a letter admitting that he loves her with all his heart and goes to find Valentine. In the ensuing fight with Valentine’s real son Jonathan (also known as Sebastian), Jace is injured and later Valentine stabs him to death. When Angel Raziel grants a wish to Clary, she asks for Jace to live again. The two are finally freed of this complicated situation now that they know that they are, indeed, not siblings. They officially begin a relationship and are happy with each other.
However, their happiness is short-lived when Jace begins to have nightmares about killing Clary. He starts avoiding her and is almost completely closed off, leaving Clary confused and hurt. He, himself, spends time with Simon who notices that Jace is physically showing signs of deep distress. When the cause of the dreams come to light via the Silent Brothers, he is asked to spend a night at the cellars in the City of Bones. Lilith possesses his mind while he’s there and even during that possession, his feelings for Clary remain unaffected. After the defeat of Lilith, Clary makes it clear to Jace that her feelings have never wavered for Jace, no matter what he did because it was the possession that made him act up.
While Clary goes away to meet the Shadowhunter team that had arrived at the scene, Jonathan Morgenstern, who was risen by Lilith as a balance to the rebirth of Jace, forces Jace into another kind of possession where they become one. Jace and Jonathan go missing, and Clary is left at the New York Institute with her friends and family trying to find him. Jace, who can still not stay away from her, comes to Clary in the middle of the night one day and asks her to accompany him. When she later realizes this is the only way she can figure out what’s going on and get Jace back, she agrees to leave with him. Traveling around Jace and Clary share a lot of their relationship’s high moments, even while Clary knows that this is not ‘her Jace’. This is confirmed later when Jace breaks out of his possession temporarily and tells her that he is jealous that ‘the other guy’ spends time with her.
When Jonathan’s gathering of Shadowhunters, who are to be administered Lilith’s demon blood, is broken up by Lightwoods and their company, Clary stabs Jace with Archangel Michael’s sword severing the connection between Jace and Jonathan and freeing him from the possession.
Later Clary comes to see Jace who is now healing in the NY Institute. The sword leaves heavenly fire inside Jace’s body and they share a kiss but he ruefully admits that they can’t do what they ‘almost did in Paris’.
Afterwards, when Clary puts her weight behind it, Jordan Kyle helps Jace deal with his simmering rage that encourages the heavenly fire to get out of control. When Jonathan, going as Sebastian, starts attacking Institutes all over the world, the Shadowhunters withdraw inside Idris. Jace and Clary stay by each other, helping each other in this tough time because they both know they understand Sebastian better than the Clave. At last they decide to visit hell along with the Lightwoods and Simon, where Sebastian is hiding, knowing that the Clave cannot defeat him, and they need to rescue Magnus, Jocelyn, Luke and Raphael. Even while having false visions of a happy life as she enters hell, Clary’s life still included Jace – she was getting married to him in the vision. She reiterates to him that she would never wish for another life because this one brought her to Jace. She helps Jace with the heavenly fire, once he loses control of it and purges him of it, enclosing the fire in her shortsword. They keep it a secret, and that secret is what helps them to defeat Sebastian at the end. It was, as Simon had thought to himself, a perfect team work.
Jace and Clary grew from bickering at each other to being two perfectly matching puzzle pieces. They return to Idris after defeating Sebastian and Jace decides to take up the name Herondale. Tessa Gray, at Jocelyn and Luke’s wedding, hands over the Herondale family ring to Clary who passes it on to Jace. He remarks that once again, she’s the one who brought a missing piece of him to him. He teases her by calling her Clary Herondale once, and their story closes with Clary and Jace sitting near the Mortal Mirror, watching the night sky with all of their friends.
By 2012, Clary seems to be officially known as a Fairchild. She, along with Jace Herondale, has taken over the New York Institute from the Lightwoods, becoming some of the youngest Shadowhunters to hold such senior positions. Both of them have affinity for the Los Angeles Institute Shadowhunters, even when they’ve realized Blackthorns are keeping secrets from the Clave.
People often comment on how they can tell Jace and Clary’s feelings for one another by looking at their faces, even all through the time when they mistakenly believed they were siblings. It’s a mark of how strongly they feel for each other for even Lilith commented on this, describing Jace and Clary’s love as one that can “burn down the world or raise it up in glory”.
As Emma Carstairs rightly observed, Clary and Jace are “an institution, a legend. They belonged together.”