Reginald "Reggie" Swire (d. late December, 1919) was a London solicitor and a widower with only one child, daughter Lavinia Swire. His brother Jonathan was Liberal Minister. It was through Reggie Swire that Lavinia met Matthew Crawley, heir to the Earl of Grantham, to whom she soon became engaged.
Once, Reginald was in debt to Sir Richard Carlisle and could not pay him back, though it is unknown why. Lavinia then went to Carlisle to plead on her father's behalf. Carlisle agreed if she would steal certain papers belonging to Jonathan, to which she agreed. The result was the exposure of the Marconi scandal.
Lavinia dies suddenly in the Spanish Flu epidemic that comes to Downton and Reggie Swire is heartbroken. He is present for Lavinia's funeral.
In the Christmas special, Matthew learned Mr. Swire was ill and went to London to visit him before he died. His dying wish was for his ashes to be buried in his daughter's grave in Downton, a wish fulfilled by Matthew, with his mother and Lady Mary Crawley present.
In the third season, Matthew is contacted by Reginald's lawyer, Mr Charkham. Reggie had three possible heirs, of which Matthew was the third. The first died of Spanish flu, before Reggie died. The second died in India visiting some tea plantations he owned there. So, this made Matthew Reggie's possible heir. Matthew is not initially happy about this and feels he cannot accept the money; he cannot "dine in splendor" as he put it, because he blames himself for breaking Lavinia's heart and taking away her will to live. Mary, Matthew's wife, supported him at first, but later tried to change his mind, because Reggie's money could save the estate of Downton Abbey, which was on the brink of collapse, because of a bad investment made by Robert Crawley.
Matthew and Mary disagreed about this for some time. When Matthew Crawley is confirmed to be heir, Mr. Charkham delivers a letter to him from Reggie. Swire had written one to each of the three potential heirs. Matthew at first contemplates burning it as he anticipates it to be filled with false praise of Lavia being lucky to find him. Mary reads it and discovered that Lavinia had written a letter to Reggie the day she died, explaining that she knew Matthew did not love her and that he was being noble, because he was prepared to sacrifice his happiness and a life with Mary, his true love, to be with her because she had stayed true to him. Mary proves to Matthew that Daisy posted this letter, unknown to the rest of the house at the time, and so Reggie knew that Lavinia would not have married Matthew even if she had lived and still he wanted him to be heir. Knowing this, Matthew invested all of Reggie's money into the estate and saved it from being sold by the Crawleys.