The Lightwoods

Isabelle was born to Robert and Maryse Lightwood She was born two years after her brother, Alec, and she was about seven when Max* was born. Around this time, her father had an affair, later finding out when she was thirteen and keeping it a secret from most everyone since then.

She was about nine years old when Jace Wayland was adopted into their family. Hearing from her mother that Jace had witnessed the death of his own father, she had expected the thin ten year-old boy to 'come to her tearfully' for comfort and advice, liking the feeling of being wanted. Instead, he seemed to not need anyone and seemed self-possessed, which was why it took Isabelle longer to warm up to Jace, while Alec was immediately delighted with him. Even at ten years old he’d had a sharp, defensive wit and an acidic temperament which, in fact, reminded Isabelle of herself. In the end it was Shadowhunting they had bonded over—a shared love of sharp-edged weapons, gleaming seraph blades, the painful pleasure of burning Marks, the thoughtnumbing swiftness of battle. She eventually felt that she loved him as a brother, especially when Jace insists upon bringing her along with him and Alec on their hunting trips.

* Max Lightwood is deceased. (1998-2007).

Isabelle has been called "a flirty, ass-kicking robot." She shares the Lightwood family's snobbish attitude toward mundanes and Downworlders. However, after meeting a few mundanes, her attitude towards them begins to change as she befriends them. She can be affected by certain, strong words suggesting she is not as cold-hearted as she allows others to believe.  Isabelle has a great sense of style and is very fashion conscious. She is also somewhat promiscuous. "Nothing less than seven inches" is her motto for wearing heels. Isabelle is portrayed as vain and narcissistic, but slightly insecure. She is sensitive about her great height and wishes that she were smaller and more delicate.  She is arguably the most mature of the Lightwood children and as such adopts something of a maternal role towards them, even her elder brother, Alec. Despite her narcissistic attitude, Isabelle is very loyal to her family, particularly Alec. She is one of the few people in the world to be aware of his sexuality and the two are very close. Isabelle also possesses a fiery temper which numerous characters are victims of. In spite of her mature and refined nature, she has a habit of throwing temper tantrums and locking herself in her room when things go wrong, flinging objects at people when they intrude on her solitude. She dates many different boys, especially boys that her parents won't approve of. She later reveals that she does this to draw attention away from her brother Alec, who is gay, so that their parents won't become suspicious of him. Her brother, Max's, death hit her particularly hard, feeling that she failed him, because she ignored him when he said something was wrong. Her grief led her to skipping his funeral, as she felt that she didn't deserve to go. Isabelle is cautious when it comes to love. This stems from her mother telling her to never trust men, because they will always hurt her. Though at the time she didn't understand the significance of it, she later revealed that she doesn't know how to truly love. She is baffled, looking at her brothers, to see them giving themselves to love. She doesn't understand how they can open themselves up to such sadness. She loves her life and the fight against demons. She greatly enjoys demon hunting. She is also often the 'bait' for demons that she, Jace, and Alec are hunting. She is a very free-spirited girl that enjoys partying, flirting, and clothes.

Isabelle is described as gorgeous. She has long, ink black hair and black eyes. She is said to greatly resemble her mother, so much that Jocelyn Fairchild-Morgenstern mistook her to be her mother when they first met. She is quite tall (5'8''), being taller than most boys, and very slim. She wishes that she were small and petite, though she is happy with her looks, because it is a hassle for her to be taller than most boys.

All Shadowhunters have some of the angel, Raziel's blood in their veins. This gives Shadowhunters the ability to survive having runes marked on their skin.

• Marks: Isabelle possess several combat marks, including healing and strength runes.
  
• Superhuman Physiology: Shadowhunters have physical abilities superior to mundanes, which are separate from abilities gained from runes.
• Superhuman Strength
• Superhuman Agility
• Superhuman Stamina
• Superhuman Coordination