The great fire caused by the contents of the Holy Grail spilling out at the end of the Fourth Holy Grail War killed both his parents and left him mortally wounded within the blaze. He was rescued by Kiritsugu Emiya , who was dejected after indirectly causing the fire and, wishing to actually "save" someone for the first time in his life, rescued Shirou by embedding Avalon in his body. He then later asked the recovering Shirou if he would like to become his adopted son, fully disclosing his identity as a magus. Shirou agreed and spent the next two years persistently asking Kiritsugu to teach him Magecraft.
34 Shirou and Rin ideas in | rin, fate stay night, fate stay night rin
The land of Fuyuki City, which possessed the second greatest spiritual power in all Japan, was owned by Rin's ancestor, Nagato Tohsaka ; at the behest of the Einzberns, he provided his property as battleground for present and future wars. Nagato was a kirishitan , as well as a pupil of Zelretch ; these titles gave him considerable influence in both the Mage's Association and the Church , allowing the Tohsaka family to participate in the Holy Grail Wars, free from harassment by either organization. Rin was brought up as the successor to her family's magecraft , and was instructed by her father to prioritize magecraft over her own interests. Ten years ago, Tokiomi Tohsaka was selected as a Master in the 4th Holy Grail War, and Rin was entrusted to take care of the Tohsaka estate until his return. Her mother, Aoi Tohsaka , was left in a debilitated state from the war, so her care was left to her guardian, Kirei Kotomine. After her father was killed, Rin continued to improve her sorcery with some guidance from Kirei Kotomine.
But he actually has several, and there are plenty of good reasons for any of them to be the right person for him. Rin Tohsaka is his would-be enemy turned friend who competes against him and supports him where she can. Either of these women would be great options for Shirou as people to pursue romantic relationships with.