Sherlock Season 4: 20 Easter Eggs & References In 'The Final Problem'
6. Victor Trevor
Things take a dark turn in The Final Problem when it's revealed that Redbeard was not Sherlock's dog at all. Eurus' real victim was Victor Trevor, Sherlock's redheaded childhood best friend. Eurus was jealous of the bond the pair shared when playing pirate games and trapped poor Victor in a well.
This is a tragic revising of a character from the canon. In Doyle's The Adventure of the Gloria Scott, Holmes reveals to Watson that he had one significant friend before him: Victor Trevor, his best pal at university.
Trevor is also indirectly responsible for starting Sherlock on the detective game, as he brought him his first case. This is much like how Eurus describes the hunt for Victor as young Sherlock's first case.