Why did Hyoudou Issei beg Rias Gremory to save Shirou? To find out what really happen, we have to do a little flash back.
Emiya Shirou was basically just an ordinary high school student who attended Kuoh Gakuen without knowing there is many supernatural things around him, his younger sister Illya was a Magical Girl and her adoptive parents were magus. Plus Kuoh Gakuen where he went to school was managed by the demon race, you could say he didn't find out not about all the magical things around him so far was a miracle.
Until the day he died because of supernatural beings. One evening when he had just returned from training at the archery club, Shirou passed the park he used to go to when he was going home. He saw his super perverted friend Hyoudou Issei sitting on the ground while holding the wound in his stomach. in front of Issei was a middle-aged man who wore a long coat and a black fedora. That middle-aged man was seen floating in front of Issei using a pair of black wings.
Shirou did not understand what was really happening to Issei. But Shirou understood one thing, his friend from the class next door were in danger and it was impossible for him to not helping to seeing his friend being injured.
"Hyoudou-san what happened to you!" Shirou shouted as he immediately ran towards Issei who was injured. "Why are you hurt like that?"
"Shi-shirou!" Issei said. "What are you doing here!"
"Your stomach is seriously injured," Shirou said. "There's no way I can just leave you Hyoudou-san!"
"Hey, human!" Said the middle-aged man in black suit. "Don't try to stop me from killing that shitty devil that you are trying to help!"
In the hands of the middle-aged man the light began to gather and the light that gathered began to form a spear.
The man threw the spear of light he made, and the spear of light struck Shirou heart.
And because of the attack, Emiya Shirou was killed instantly.
"Shirou!" Issei shouted.
"That annoying human is dead," Said the man. "Now it's your turn shitty devil!"
Once again the middle-aged man made a spear of light and was ready to throw it at Issei, but before he could do it ...
"Stop that!" Rias shouted while attacking the middle-aged man using her magic.
That man didn't attack Issei because he had to avoid attacks from Rias.
"Deep red hair, you must be from the Gremory Clan," said the man. "Why did you stop me!"
The man looked at Rias with anger and hatred as if he really wanted to kill Rias.
"The person you want to kill is part of my group," Rias said. "Mr. Fallen Angel, if you try to kill him again then I won't stand still!"
"So that kid is part of your group? Then I'm sorry, I thought he was a wild demon moreover I completely forgot that this city was the territory of Gremory and Sitri. My name is Donaseek, then daughter of the Gremory Clan. I excuse myself! "
The man bowed his head to Rias and flew away from Issei and Rias.
"Are you ok, Issei?" Rias asked while walking towards Issei.
"Okay is not right word Buchou," Issei said. "My stomach bleeding pretty badly, and Shirou my friend died because he protected me."
Rias looked at Shirou whose body was lying rigid on the ground. On Shirou's left chest there was a large hole due to an attack from Donaseek. Blood flowed from the hole and made the ground around Shirou become full of blood.
"He is in the class next to you, right Issei," Rias said. "Why did he die protecting you?"
"Shirou happened to pass by this park when I was attacked by Donaseek," Issei said. "And because he was trying to help me who was injured. Donaseek instead targeted him and killed him!"
"He's a very good person," Rias said. "If I were in his position maybe I would have run away and saved myself."
"Shirou is indeed a very good person," said Issei. "He even befriends me, a super perverted man who like a woman breast and hated by almost all the women in Kuoh Gakuen."
Issei then knelt before Rias and lowered his head to the ground while saying.
"Rias Buchou, please save Shirou!"
Flashback end.
"Haaah," Rias said. "Looks like I don't have much choice, especially since he helped you earlier so I thought maybe it was okay to save his life because he helped my servant. "
Rias takes out her evil pieces and prepares to save Shirou's life. Rias first tried the knight pieces on Shirou the evil pieces got in but Shirou's situation hasn't changed at all, Rias panicked then she put the rest of his 7 pawn evil pieces she had left into Shirou's body and this time Shirou's condition began to improve the hole in his left chest closed and his body that had been look pale starting to become better.
"This is truly extraordinary!" Rias said and she looks really happy. "Thanks to you Issei I got a Jackpot!"
"Buchou what happen?" Issei asked.
"To save Emiya Shirou's life, I need one Knight and seven pawn pieces," Rias said. "That means he has enormous potential!"
"Eh, does that mean you can say that the potential of Shirou is far beyond me?" Issei asked.
"Well, that's how it is," Rias answered. "To revive you at that time I used 1 mutation pawn which is equivalent to 8 pawns and by coincidence all my pawns pieces are mutation pieces! If I add up all of my pawn, it's equal to 64 pawn! you use 1 piece means your ability is equivalent to 8 pawn while for Shirou he uses the remaining 7 pieces and knight which is equivalent to 5 pawn then total pieces I should need to save him are 61 pawn! I'm lucky all of the pawn pieces from my evil-pieces are mutations, so I can save his life! "
"Wooow," said Issei. "I'm really surprised at how much potential Shirou has."
"Yup!" Rias said. "Either his Sacred Gear or the power he have so that I has to use many evil pieces to help him, but it's clear that I got him in my peerage is an incredible fortune!"
Meanwhile with Emiya.
(That girl put something called evil pieces into my body and obviously my physical body changed completely because of the evil pieces, I could feel my magic circuit which had only amounted to 27 units with class C increased to thousands of times more with what class! Ex? Ex class? does it feel like the impossible from c to Ex? I don't think this magic circuit is from the evil pieces. But this magic circuit is a gift from God to me and evil pieces are only intermediaries to activate it! And again besides the active magic circuit. I can feel a pair of black wings at the top of my waist, is this the effect of the evil pieces?). Emiya tried to wake up and think longer but his consciousness gradually disappeared and in the end Emiya was completely unconscious due to the injuries he suffered plus his change to the devil race made a great toll in his body.