Chelsea (Lexie Grey's long lost daughter)

After a search and rescue mission for a 13 year old girl goes successfully, she ends up at Grey Sloan. Meredith unable to find her parents, does a D.N.A test only to discover the girl is her dead sister's daughter that her sister had a year before the plane crash and did not tell anybody she had one because she couldn't financially keep a baby. No one knew, not even Mark.