Steven Carter

Steven Carter was the son of Joe and Alice Carter and the grandson of Jack Harkness and Lucia Moretti, therefore Gray's great-nephew.

Biography
Steven's mother described Jack as his uncle as she wanted not to explain to him Jack's immortality and agelessness. The two appeared close, although their contact was infrequent. (TW: Children of Earth: Day One)

Steven and Alice were taken as hostages by Agent Johnson and her team in order to keep Harkness from making public his knowledge of the 456. (TW: Children of Earth: Day Three)

As Jack needed a child to defeat the 456, Steven was used to reverse the signal and kill the lone representative of the 456 on Earth, without explaining the situation to Steven.

By reversing the signal, Steven, as Jack knew he would, died. His mother cradled his dead body tearfully as Jack watched. His mother, Alice, did not seem to forgive Jack for sacrificing her son. His death was also the final factor in Jack leaving Earth; the deaths of Suzie, Owen, Tosh, Ianto and Steven had become too much. (TW: Children of Earth: Day Five)

Behind the scenes

 * Steven's death was the first death of a child seen onscreen in the Doctor Who Universe. However, DW: The Chase saw a baby thrown overboard that can be presumed to have died.