Reginald "Reggie" Swire (d. late December, 1919) was a London solicitor and He was 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, whom she soon became engaged too.
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.
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. Reginald has named him his heir, which Matthew refuses because he still feels guilt about Lavinia.