Sanjay (Death of the Doctor)

Sanjay was Luke Smith's husband and Sarah Jane Smith's son-in-law. (WC: Farewell, Sarah Jane)

The pair met at Oxford University. Early on, Luke described Sanjay as "the best mate I've ever had all my life." He was down the hall from Luke at Oxford. 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) They both worked for UNIT together. (TV: The Three Doctors)

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. However, this never came to pass on television, due to Elisabeth Sladen's passing.

Eventually, it would come to pass when the webcast Farewell, Sarah Jane was released in 2020, where Luke mentions that he and Sanjay have been married for five years.