Name: Lord John William Bertram Armstrong Grey
Born: 1729 (he is 16 in 1745)
Parents: Duke of Pardloe and Benedicta Grey; step-father Sir George Stanley
Siblings: Harold and step-brother Percy
Spouse: Isobel Dunsany and ?SPOILER-ALERT?
Lord John first appeared in the story two days before the Battle of Prestonpans when he was a 16 year-old scout for the Redcoats. He attacked Jamie in the woods and was captured. Jamie broke his arm, and tricked him into thinking that Claire was a captured English lady who was being ravished by him and his men. Instead of killing him, he was tied to a tree where the English were likely to stumble on him. John (who told Jamie he was William Grey) said he owed Jamie a debt of honor, which he told his brother Hal about. That just happened to be a very lucky thing for Jamie almost seven months later when he was badly wounded at the Battle of Culloden! John's first love, Hector was killed the day after he met Jamie at the battle of Prestonpans, although An Echo in the Bone erroneously says it was at Culloden.
Later on when Lord John is appointed the governor of Ardsmuir Prison where Jamie happens to be a prisoner, it is revealed that Lord John is secretly infatuated with Jamie, who is EXTREMELY not interested. It gets in the way of their friendship, which develops over several decades. As long as John keeps his feelings for Jamie strictly contained, they're good. John manages to get Jamie paroled to his family friend's estate at Helwater, in the Lake District, where he encounters Lord Dunsany's head-strong daughter Geneva. John proves again to be a good friend to Jamie by stepping in to take care of Jamie's and Geneva's son William when has has to leave Helwater, by marrying Willie's aunt Isobel.
Their adventure continues throughout all of the books after Outlander.