Name: Nightshade

Isn't that the berry in Hunger Games? I like it. 

Age: 18 moons 

This is a very popular age. But it's cool. All cats are this age eventually.

Appearance: sleek, narrow-shouldered black she-cat with shiny fur, small paws, a long whip-like tail, and jay-blue almond-shaped eyes

Wow. Maybe shorten it to be something like Nightshade- narrow shouldered black she-cat with long tail and blue eyes? Lol. I like her.

Clan: ThunderClan (yes, I knew very cliché)

It is. But we know most about them so it's fine.

Rank: warrior

That's cool.

K/C/M: no

You should make a tom called Sunfang, or Lightpelt, or something. They should have kits and rule the world.

Personality: hot and cold, quiet, sarcastic, untrusting, patient, but completely dedicated, loyal, and loving to cats she sees as her friends

I like her. She is pretty cool.

Backstory: she and her sister where born to a rouge who couldn't take care of them. So she brought them to another rogue she-cat who agreed to take care of them. After a few moons and a tragic death the rogue family want to leave, but Nightshade and her sister don't. The rogue family leaves and the sibling go to find a place to stay, away from their old home. They accidentally cross the border and caught by ThunderClan warriors. Bramblestar (the least at this time) lets them join much to Nightshade's dislike. Though she doesn't like it her sister loves clan like a lot so she stays. They soon become apprentices and warrior with the help of their mentors. One very calm, the other very distrusting. But they later become warriors their clan can be proud of.

The rouge-turned-clan-cat cliche is kinda overused, but I like how you did it. Nightshade seems like she had a pretty tragic life, but she seems like the kind of cat I should meet.

Overall, I give her a 10/10. There are some flaws, but Nightshade is awesome. ;)