Dating gang members
The incident devastated Arthur and he and he never truly coped with the loss.
Arthur mentions Isaac when conversing with Rains Fall, telling him that the incident made him realize that he can't expect good things to happen to him with the life he led.
They were smitten with each other but prevalent circumstances, such as Arthur's loyalty to the gang and the disapproval of the relationship by Mary's family, particularly her father, caused their relationship to fall through and they wouldn't see each other again for a long time. Mary and Arthur reunite when she sends him a letter to come meet her outside of Valentine, and asking for his help in getting her brother away from Chelonia, and later again in Saint Denis, this time with her father.
Not much is known about Arthur's relationship with his father, though the little information given implies a strained one at best.
In regards to his death, Arthur says that he watched Lyle die and "it weren't soon enough," referring to him as a "no good bastard" in a journal entry after his tuberculosis diagnosis.
He later learned that they were robbed and killed for a meager amount of money.
The incident hardened Arthur ever since and he never truly coped with the pain.