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. (TV: Death of the Doctor) Sanjay studied biology. (PROSE: Death of the Doctor)

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 end 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)

By the time Sarah Jane died, having told Luke that Sanjay was "looking at [him] in that way", the twosome had been married for five years. (WC: Farewell, Sarah Jane)

Behind the scenes
While the The Sarah Jane Adventures was still in production, the plan was that Luke would have come out as gay and Sanjay would have been his boyfriend. Sadly this never came to pass on television, due to Elisabeth Sladen's passing.

The plan did eventually come to pass in the webcast Farewell, Sarah Jane, penned by Davies, in which it is revealed that Luke and Sanjay married.