Edward Theodore Parker best known as Teddy Parker was a close friend and brother in arms of Booth's who served with him in the army. He was shot down and instantly killed at Booth's side during a snipe at the age of 20 and became the namesake of Booth's son, Parker. When Booth is later revealed to have a brain tumor ('The Critic in the Cabernet'), it's left up for debate. A popular counter argument is that Brennan's encounter was seen from Booth's perspective and Booth had projected Teddy onto a random soldier.