Sanjay (Death of the Doctor)

Luke Smith described Sanjay as "the best mate I've ever had all my life." He was down the hall from Luke at Oxford University. Luke said they clicked straight away; "He's smart, he's cool, he keeps making me laugh." Clyde Langer was jealous upon hearing of his new competitor.

At some point the two made a bet that Sarah Jane would ask if they'd finished their work — two thousand words on pendulum physics — before going out for a burger with everyone else. Sanjay therefore owed Luke five pounds. (TV: Death of the Doctor) Luke later had to retire from a conversation with Sarah Jane and Rani Chandra in Ealing because he'd promised to meet Sanjay at the Union. (TV: Goodbye, Sarah Jane Smith)

Behind the scenes
If The Sarah Jane Adventures had continued on, Luke would have come out as gay, and Sanjay would have been his boyfriend.