At first, she is described as "freakishly gloomy" and "doesn't speak". She is anti-social, clumsy, observant as well as honest, and funny.
As time goes on her personality begins to change. She now accepts people as friends more easily. She also becomes more social and more attached to people. This is proved when Megumi befriends Asami and Aiko in order to leave Mei without friends. Despite this changes in her personality, Mei continues to be as clumsy, observant and honest as she was in the beginning.
Mei also begins to help people who have are in situations similar to the ones she has experienced and tries to give out advice every now and then. She has become more confident and is willing to express what she has to say.
When Mei was little she had friend who she trusted. When the classroom pet died, her 'friends' turned against her and blamed her for the pet's death. However, Mei was not responsible and the true culprits were her 'friends'. This experience greatly impacted Mei's life as she learned that people turn against each other, therefore she shouldn't trust.