Theresa "Tessa" Herondale (née Gray) is the half-Shadowhunter, half-Eidolon demon who gets herself entangled in the lives of the Shadowhunters at the London Institute during the Victorian era.
Early Life
Theresa Gray was born to the unmarked Shadowhunter, Elizabeth Gray and an Eidolon demon, but raised by who everyone believed to be her father, the mundane Richard Gray. Under normal circumstances, children of Shadowhunters and demons would be stillborn, but Elizabeth was able to give birth to Tessa not only because she carried no runes, but also because she wore the clockwork angel.
Tessa, a sixteen-year-old American warlock in Victorian-era England, spent the majority of her life being overprotected by her aunt, Harriet Moore, after the death of her parents, Elizabeth and Richard Gray, in a carriage accident when she was three years old. She lives almost all her life in books, dreaming of having the kind of adventures the heroines she reads about do. Her Aunt Harriet dies from a fever, but Nate reveals that he sent her poisoned chocolates, knowing that Tessa would give them to her Aunt to eat.
For much of her early life, she was unaware of what she truly was and her true parentage.
Clockwork Angel
Tessa is forced to travel from New York to London when her brother, Nate, calls for her, shortly after the death of their Aunt Harriet. Once there, a pair of mysterious and sinister women, whom she calls the Dark Sisters, give her a letter written in Nate's handwriting and tell her that they've come to collect her on Nate's behalf.
The Dark Sisters reveal that they have Nate as a prisoner, showing her their father's ring, that Nate always wears, splattered in blood. They force her to Change, or shape-shift into anyone, with the threat that they will harm Nate if she doesn't do what they say. When Changing, she also touches the mind of that person and can access their memories and thoughts. The Dark Sisters hoped to exploit her ability and sell her into marriage to "the Magister". In her isolation, she writes letters to Nate about her loneliness and imprisonment, which Will later finds, and reads.
One day, after Changing into Emma Bayliss, she is told that she will be married to the Magister, who has long been waiting for someone with her ability to come along. After a futile attempt of escape, she meets Will Herondale, who actually trespassed into the Dark House in hopes of seeing anyone that might help in the Shadowhunters' investigation into a 'ouroboros' dagger that was found next to Emma Bayliss' dead body.
Tessa escapes from the 'Dark House' with Will, whom she befriends. However she passes out whilst in the midst of the fight against the Dark Sisters and is taken to the London Institute.
She befriends Jem Carstairs, and the rest of the Institute's residents and staff. When the investigation into her brother leads them to Alexei de Quincey's house, she takes the form of the vampire, Lady Camille Belcourt to sneak in, with Will posing as her human subjugate and Magnus Bane, Camille's lover, as her guide. Once the human captive at the party is biten, Will signals the Nephilim to come in and kill the vampires. Just as he does so, Tessa realizes that the captive is her brother. After the battle, they take him to the Institute.
During the battle, Will is attacked by de Quincey and bites the vampire's arm, inadvertently ingesting some vampire blood, which threatens to turn him into a Darkling. He must drink holy water but refuses it from Sophie. Tessa takes it to him, up in the attic, and he drinks it and then kisses her, but then shoves her away and demands that she leave him alone. After this he is standoffish and unfriendly, much to Tessa's confusion.
When confronted by the Magister, Axel Mortmain, Tessa recalls Will's words to her earlier, explaining that the British warrior-queen Boudicea was braver than any man because she killed herself rather than be taken by the Romans. With this inspiration, she prepares to kill herself to escape Mortmain. However, she realizes that if she Changes into the form of a dead woman whose form she had already taken while under the tutelage of the Dark Sisters, she will bleed convincingly. By changing very quickly, and for a very short time, into a woman who had been shot, she convinces Mortmain that she is dead and he leaves when confronted by Will.
Will is momentarily affectionate, but then puts an even greater distance between them. At the end of Clockwork Angel, she confronts Will, hoping to reconcile, but he is unkind, treating her like a prostitute and reveals that she is unable to have children as she is a warlock. Tessa leaves, hurt and confused.
Clockwork Prince
Tessa is present, and is a key witness, at the trial of Charlotte Branwell in front of the Clave, about the attack on the London Institute. Charlotte is given two weeks to find Mortmain or she will have to resign as head of the Institute, and be replaced by Benedict Lightwood.
Tessa, accompanies Will and Jem, up to the York Institute to meet Aloysius Starkweather, who may hold a clue to the Magister's past. Jem gives her the Carstairs family ring, so that she can pretend to be a mundane girl, who is engaged to Jem.
Tessa is trained by Gabriel Lightwood to defend herself, in case there was an attack against the London Institute.
Sophie Collins comes to Tessa with an invitation to a masked ball at the Lightwood's house, addressed to Jessamine from Nate. Jessamine slapped Sophie, so Sophie hit her on the head and knocked her out. Tessa sends Sophie to Will for help, and he places a sleeping rune on Jessamine so that she will not wake up until morning, and ties her to her bed as a precaution. Tessa Changes into Jessamaine, and Will accompanies her, so that she can sneak into Benedict Lightwood's house. Tessa meets Nate, who dances with Jessamine, and reminds her that she is now 'Jessamine Gray'.
Jem proposes to Tessa, and gives her his jade necklace as a token of their engagement. However, Will arrives back from Magnus' house after the demon, Marbas, reveals that there was no curse on him. He talks to Tessa in private and declares his love for her, but she rejects him and tells him that she is engaged to Jem.
Clockwork Princess
In Clockwork Princess, she is revealed to be a half-Shadowhunter, half-Demon, a new species, because her mother Elizabeth Gray, the real Adele Starkweather, was a Shadowhunter who bore no runes, and her father was replaced by an Eidolon Demon, making her a hybrid and an immortal.
She is kidnapped in a assault on the Institute at the delivery of Jessamine from the Silent City where she is taken to The Magister, who has taken up refuge in Cadair Idris near Will and Cecily's former home. Will then came and found them, but his long trip was to no avail as he is quickly captured by his own stupidity and Mortmain's wall. The rest of the Enclave then shows up, accompanied by three Silent Brothers: Enoch, Micah, and Zachariah.
In the fight with the Infernal Devices, it is revealed that her fiancée, Jem, has become a Silent Brother, Brother Zachariah. Since Silent Brothers cannot marry, they are separated.
Tessa eventually ends up marrying Will, and the two stay together for at least 50 years. Tessa stays with Will until he grows old and passes away. After Will's death, Tessa keeps her distance from her family, not wanting to go through the pain of seeing her children and other loved ones grow old while she remains youthful.
More than 100 years later, Tessa meets with Jem who has found a cure for his prior addiction to Yin fen and ascended from the grip of the Brotherhood. They quickly reconcile, ending the book on the Blackfrier Bridge, kissing and agreeing to travel the world together.
