Zack Martin (Galaxies at Wars)

Zack is a character who doesn't really appear in the main story itself, rather he appears in many flashbacks involving Johns past. He and John met in a train where they both were to join the military as mercenaries. They quickly became good friends and fought their enemies along each other. Their friendship was a very strong one and lasted several years till Zack died, after being shot by a soldier from Iscariot in the back. 
At the side of his dying friend, John swore to live as Zacks legacy and to live a happy life in his name. Not knowing if this would be possible, John still promised to do as Zack told him. 
His death affected John deeply as he kept fighting with Zacks gunblade for years till it got broke. After the gunblade was rendered useless, he returned to Zacks grave and put it gently on Zacks grave to honor him. 
Scar used Zacks death against him, when he tried to possess John. He öplayed on all his insecurities and made him feel guily about Zacks death. This nearly broke John if he didn't got saved by the thought of his promise to Zack and by the help of his closest friends. 
Zacks death was so traumatizing to John, that the latter went on a killing spree, killing everyone who has involved in killing Zack. After killing Emilia, John created his memories anew, making himself believe, that Zack was killed by hostile soldiers, that Emilia committed suicide even though John killed her and that Zacks weapon was his own.