答案：My Best Friend
My best friend is Lily. She is a kind and outgoing girl with a big smile. She is always ready to help others and has a positive attitude towards life.
We became friends when we were in Grade Seven. One day, I lost my math textbook and was very worried. Lily noticed my anxiety and offered to share her textbook with me. She even spent extra time explaining difficult problems to me after class. Her kindness and willingness to help touched me deeply. Since then, we have been good friends. We often study together, share our happiness and sadness, and support each other in everything. I am lucky to have such a good friend.
解析：文章介绍了最好的朋友Lily，描述了她善良外向的性格，以及因为Lily帮助自己解决丢课本的困难而成为朋友的经历，符合题目要求，语句通顺，词数达标。