By 

Name: Lilac

Lilac is such a pretty name, I really like it.

Gender: she-cat

She-cat is a little bit overused, but I'm guilty of this as well.

Clan: RoseClan

Family:

 Mother: Orchid-the 'Ivy' of the group (rule enforcer, overseer, and healer). A dark gray she with forest-green eyes. Her pelt carries a scent of rosemary and her eyes carry a soft, warm light. Her fur is short and she has a stubby, thin tail. Her build is bulky and her voice is firm but soft. She cares about everyone a lot, but she could be too overprotective and panicky. She can also be overly bossy.

This is really awesome; I love that you made a new Clan role. 
Sister: Plumeria, a black she with a lean build. Her fur is is medium-long with friendly, forest green eyes that carry a tint of curiosity. Her fur smells of grass and she has a short, fluffy tail. Her voice is cheerful and optimistic, with a bubbly tone to it. She's overly friendly and bubbly. However, this leads Plumeria to be nosy and naive.

I like that you describe their scents, too. Plumeria sounds a little like my sister, to be honest. (The personality part, not the physical description, obviously)
Father: Unknown, never mentioned because "a sore subject" for Orchid when asked.
Brother: Strike, died at birth, a tiny black furball with a sturdy build and a thin, long tail

Personality: Lilac is a stubborn she-cat but is determined to reach justice. She's rebellious against cats who are above her and think they can just use her. She is extremely prideful-to the point where she could flaunt her abilities, which isn't exactly the best thing to win other cats over. She's willing to kill in revenge-but only if it's fair, not personal bias against some cat who "stole her prey". She's cold, calculating, and strict; but shows a soft side as well. She's brave and rarely shows feelings, but instructs discipline and equality among the cats of the Clan. She doesn't show favor to any cat and is diligent, keeping her Clan together. She gets easily offended as well and overly defensive. She's also quite impatient sometimes and tends to be sassy and sarcastic to cats who object her every order and boast about how strong they are. She can also be manipulative when she wishes to ensure her own desires.

This is a really interesting cat. I like the 'bad' traits like cold, manipulative, etc. I kind of think that the 'not showing emotions' and the 'easily offended' parts clash a little, though.

I rate Lilac a 6/10. Not because she was a less-awesome OC, but because I was really looking forward to knowing her backstory and it wasn't included. I like the cat descriptions and the personality, though!