Shareen Costello

Shareen Costello was a friend of Rose Tyler's.

Profile
When Rose was in school, she would often intentionally miss class to go shopping with Shareen and the two girls would go to look at boys. (DW: The Unquiet Dead)

Before Rose left her home planet of Earth with the Ninth Doctor on 6 March, 2005, Shareen had been upset and Rose had tried to comfort her, spending at least one night at Shareen's in her effort to do so. (DW: Aliens of London)

Also before leaving, Rose had heard Shareen, on at least a few occasions, state, "don't argue with the designated driver". (DW: "The End of the World") Rose quoted Shareen's statement after journeying with the Doctor to Platform One in the year 5.5/Apple/26. (DW: The End of the World)

After Rose returned to Earth with the Doctor, she told him that she planned to explain her twelve hours' absence from the council flat she lived in with her mother, Jackie, by telling her mother that she had spent the night at Shareen's. When she went back to her home, Rose claimed that Shareen had been "all upset again". However, as Rose had actually been away for twelve months, and not the twelve hours that the Doctor had erroneously estimated, Jackie knew better than to believe her story about having been at Shareen's. (DW: Aliens of London)

After an argument with Sarah Jane Smith, Rose cites that the only time she and Shareen "fell out" was over a man, and now she and Sarah Jane are similarly fighting over the Doctor, implying some level of friendship with her.

Behind the scenes
Shareen has never appeared on Doctor Who and has only ever been mentioned by Rose.

In the Doctor Who Annual 2006, an article about Rose, written by Russell T Davies, includes a paragraph that gives Shareen's last name as "Costello". The paragraph details how, on a school trip to France when Rose was 13, she and Shareen, Rose's "best mate" (at least at that time), escaped the teachers who were accompanying them, crossed town, and boarded a train to Parc Asterix, instead of going to the Louvre (which they had been scheduled to visit on that day). After they were caught by police while waiting in a queue for the Menhir Express, Shareen was sent home with Rose.

Rose's virtually constant travels with the Doctor and the less frequent references to Shareen in episodes of Doctor Who after "Aliens of London" suggest that Rose began to have less contact with Shareen. Yet, if this is true, the diminishing nature of their relationship is merely implied and is not otherwise established.

Russell T Davies would later gave Torchwood character Suzie Costello a similar name.