Sophia Morton is the girl who committed suicide at the club Thomas was working at in the fourth series.
Sophia committed suicide after buying drugs off of Naomi in the club Thomas was working in. Naomi had known her but told the police who were interviewing people that they never met. Emily and Naomi were listed as her close friends on a list from Sophia's parents but Emily had never met her before.
Emily then started to investigate Sophia and went to Sophia's house to talk to her parents. Sophia had claimed that Emily and Naomi were her best friends. Emily was looking around Sophia's room and discovered that Sophia was a lesbian. She also found a wooden box and a University prospectus with a key inside it. Emily began flipping through a University prospectus when she found a picture of Sophia and Naomi laughing at a University Open Day.
Emily slammed the photo on Naomi's classroom window, interrupting her class and the two start to argue. Naomi admits that she lied about Sophia and that she did meet her at the open day but they didn't do anything.
Emily suspects that Naomi is lying but shows her the key she found at Sophia's house. Naomi believes it is they key for her locker at the Army base. They go and open the locker, where they find a shrine dedicated to Naomi.
They next day, Emily goes to the club where Sophia died to meet her brother, Matt, to find out what is in the wooden box. Naomi turns up and begs Emily to leave it, but Emily says she can't. They open the box and find Sophia's sketchbook that details Sophia's one day affair with Naomi as Naomi did in fact cheat on Emily.
Cook visits Naomi and tells her that they have to deal with the guilt. Cook then tells the police that he sold Sophia the MDMA, taking the fall for Naomi. He pleads guilty at his trial and is sent to jail.