Meet Tough Cookie :3
 
She is a student (18 years old) at Twilight Sparkle’s School of Friendship. Unlike a lot of the other students, she is pretty miserable and cynical. She doesn’t pay attention or care about her classes, and she keeps getting a ton of bas grades as a result. She is often pretty snarky and uncaring to a lot of the other ponies.
 
The reason she is like this is because she has a bad home life :c  
Her parents where in a loveless marriage, and they didn’t care about their daughter due to her being unexpected. They where uncaring about her, and eventually they kicked her out after one too many arguments, resulting in her having to live by herself.  
This resulted in her attitude, and how she met her future best friend and lover, Cheery Candy.
 
Seeing the sad mare, Cheery tried to cheer her up, only for Cookie to keep being mean to her. Despite this, she still persisted, and after asking why she was like this, and hearing why, Cheery vowed to make sure she becomes a much happier mare, and the two became friends.
 
Cookie also has a secret love for cooking and baking, hence the name. She hopes to one day have a baker of her own.