Cassandra Clare shared these descriptions of some of the characters from THE DARK ARTIFICES:
Helen is described in City of Lost Souls. White-blond curly hair, blue-green eyes (those eyes are a hallmark of the family), petite, slightly pointed ears due to faerie blood in the family.
Mark: Same white-blond curly hair, but he has one blue eye and one gold. Tall, thin, has the same faerie blood as Helen, so rather delicate bone structure. Favored weapon: curved, single-edged blades like katanas.
Julian: Dark brown messy hair, blue-green eyes. No faerie blood here or in the rest of the family on down — he’s only half brother to Helen and Mark. Different mothers. He’s a little lanky, with high cheekbones. Bites his nails. Usually splattered in paint. Favored weapon: throwing knives, seraph blades. Seventeen years old.
Olivia: Brown hair like Julian, the same blue-green eyes. Long lashes and a cupid’s bow mouth. Where Tiberius is extraordinarily weird, Livvy is extraordinarily ordinary, at least on the surface. Underneath she’s peculiar as her brother, but in a very different way. She and Tiberius are nine months younger than Jules. Favored weapon: poison darts.
Tiberius: Manages not to look like anyone else, which Julian puts down to sheer stubbornness. Black hair and gray eyes. When he and Livvy were born they had the same gray eyes, but hers changed and though the family were sure his eyes would also change color from gray to blue-green they never did. Has the same long lashes and cupid’s bow mouth as his twin sister though. Favored weapon: Livvy will kill you if you look at him funny, so why bother?.
Drusilla: Is only thirteen. She has the Blackthorn brown hair and blue-green eyes, but she is the only one of them to have freckles. Most of the Blacthorns are lanky and tall, but Silla is compact and will grow up curvy and short.
Octavius and Ariadne are so little — Tavvy is only six, and Ari five — but they both have the Blackthorn dark brown hair and blue-green eyes.
Emma has golden-blonde hair which she keeps long for sentimental reasons, but usually has braided back or up in a bun for efficiency’s sake. Her eyes are dark blue. She works extraordinarily hard at her training – her body is strong and tough, and she has a bad scar that runs up her right arm, that she’s both proud of and a little self-conscious about. Telling blonde jokes in front of her is a good way to find yourself in the hospital. “Oh dear, did I just drop that giant broadsword on your head? I guess I did! Must be cause I’m blonde.” Favored weapons: the repeating crossbow, wind-and-fire wheels.
Cristina is Hispanic; she’s from Mexico City and is one of the few Institute kids who has a living parent. She’s got gorgeous black hair, though she keeps a long swatch of it dyed red, and dark brown eyes. Where Emma has a temper, Cristina generally gets angry only if you hurt someone she cares about. Then she smiles sweetly and plans to kill you in your sleep like a ninja. Emma was thrilled when Tina came to the Institute because it meant a girl for her to be friends with, and they’re very close. Favored weapon: chain weapons like surujin.