I'll go first.

Name: Ali
Age: 14
What are you? (Shadow hunter, mundane, warlock): Shadowhunter
Weapon Of Choice: dagger
Personality: sarcastic, funny, jokes around a lot, but knows when to be serious
Looks: frizzy red hair, golden amber eyes, small spiral shaped birthmark under her left ear
Backstory (Optional): woke up in the institute one day with no recollation of her past
Other:

Here's mine

Name: Carrie
Age: 13
What are you? (Shadow hunter, mundane, warlock): Shadowhunter
Weapon Of Choice: Seraph Blade
Personality: sarcastic, smart, crazy, weird
Looks: long straight blonde hair, green eyes, tall
Backstory (Optional):
Other:

I actually have twin OC ^^
Name: Halanore
Age: 17
What Are You?: Shadowhunter/Angel
Weapon Of Choice: Blade
Personality: Serious, No-nonsense, Never laughs or cracks a smile
Looks: Short Bob Hairstyle, White hair (Dyed) A Bit Tall, Some muscle, Triangle tattoo on forehead and a lighting bolt tattoo that runs down from the triangle to her arms and legs, Red eyes
Backstory (Optional): She and her brother are dead but keep their spirits around. She is keeper of light and her twin brother is keeper of darkness. They have a combo power when united cause a huge power wave.
Other: She is blind in her left eye

Name: Darach
Age: 17
What Are You?: Shadowhunter/Demon Angel
Weapon Of Choice: Sword
Personality: Fun, Cheery, Jokester, Prankster
Looks: Medium Flipped Back Hair, Black, A Bit Skinny, Circle Tattoo on forehead and lighting bolt tattoo that runs down from triangle to his arms and legs, Bright blue eyes
Backstory (Optional): (Same as Halanore's)
Other: He is blind in his right eye

Name: Kaity
Age: 16
What are you?: Shadowhunter
Weapon of choice: various, anything available
Personality: Witty, sarcastic, girls-are-better-than-boys kind of thing.
Looks: Tall, slightly muscular, golden-brown hair (almost always in a ponytail), dark green eyes, small star-shaped scar on right shoulder.
Backstory (Optional): she was captured and tortured for information by demons and now trusts no-one at all, she is very closed to others and her past remains a mystery to all.
Other:

MINES A TWIN TOO!!!

Name: Liza
Age: 18
What are you?: Shadowhunter
Weapon Of Choice: Anything that is completely dangerous and pointy
Personality: Boyish, A genius, Funny, Depressed at some times
Looks: 5'8'', Black hair, BRIGHT blue eyes, 36-24-36, pretty, Has a large scar at the back of the shoulder.
Backstory: she dated a shadowhunter, without the Clave knowing it, she thought that he would be her parabatai but let her down by dating a mundane. She became depressed, year by year, she waited for the boy to come back.
Other: Loves killing mundane wrong-doers

Name: Lawrence
Age: 18
What are you?: Shadowhunter
Weapon Of Choice: Samurai
Personality: Quiet, observant, Confident, Sarcastic
Looks: 5'9'', Black hair, blue green eyes, muscle-y, good looking
Backstory: Because of his twin sister's depression, he offer himself to her, to be come a life long parabatai, Liza was happy. But introduced a consequences, Liza will be his protector on fierce and intelligent demons, the a kinds of thing Lawrence would never fight.
Optional: Is good in making friends, mundane, shadowhunter or a warlock