"The effect of Oath Under the Snow allows you to add one Miyu Sakatsuki from your Deck or Graveyard to your Hand and Special Summon one Emiya Servant Card (excluding Miyu) from your Deck to your field. The Servant summoned by this effect cannot be destroyed by battle until the end of the turn!"
This effect corresponds to the scene where Shirou Emiya (Oath under Snow) protected Miyu until the very last moment of her Spiritron Transfer.
"I'm summoning Shirou Emiya (Oath under Snow)! (It's essentially just an alternate art of Shirou Emiya, distinguished by this name from the Fifth Holy Grail War Shirou.)"
Shirou Emiya (Oath under Snow): Attack Power 1600
Identical to Shirou Emiya in every way, Oath Emiya (hereafter referred to as Shirou and Oath) appeared before Miyu. Upon closer inspection, Oath's eyes held a more resolute glint, befitting a man who had witnessed his beloved junior's death and abandoned the world to save his sister.
"Julian, what tricks are you playing now?!"
Having suddenly regained his freedom and his body restored, Oath immediately adopted a combat stance. He knew that Julian's actions must conceal some sinister motive. But he quickly realized that none of the people around him were familiar, and directly opposite him stood someone who looked exactly like himself.
"Brother!"
Miyu was certain this was her real brother—the one who had been by her side for years.
"Miyu... Didn't I wish to send you to a world where you'd be safe from harm?!"
Seeing Miyu behind him, Oath Emiya tensed. Had Julian somehow captured her again? So wishes are completely unreliable! The only thing I can rely on is myself!
Eresh, secretly observing from a corner, couldn't help but marvel at how humans could display such a rich range of emotions in just a few seconds.
"This is another world. Welcome, Mr. Shirou Emiya (Oath under Snow)."
Jiang You's hand had already reached for the light switch when Illya glared him back. While the Prisma Illya world faced an imminent threat of annihilation, the combat power Jiang You had at his disposal far exceeded that world's limits. What was Pandora's Box and its world-consuming Black Mud? Did it even compare to Tiamat's Primordial Sea? Jiang You was simply enjoying himself.
"Why are you here? Your mind should already know."
"I see."
Oath Emiya nodded. What a convenient Noble Phantasm. He also felt a twinge of relief that his Holy Grail War hadn't summoned Jiang You; he wasn't confident he could have defeated him.
"Thank you for helping my sister and me. I'm trapped and unable to repay you. But even so, I must humbly ask for your assistance. Miyu can't take care of herself yet. Mr. Jiang You, please help her!"
Oath Emiya couldn't be certain of Jiang You's friendliness, but he had no choice. Seeing Miyu again at this time was already a stroke of luck. He wasn't overly worried, though. The servant known as Jiang You was immensely powerful; if he harbored ill intentions toward Miyu, he wouldn't need to feign anything. Oath Emiya could see that nearly all the servants here were centered around Jiang You. Such overwhelming strength would make protecting a single person child's play. As long as Miyu's identity as the Divine Child remained concealed, she would be perfectly safe.
"I'm sorry, but I refuse!"
Oath Emiya looked as if he were entrusting Jiang You with his final wish, nearly making Jiang You laugh out loud. Julian would never harm Oath Emiya to threaten Miyu, as proven by the plot itself—Oath Emiya couldn't possibly die.
"...If you're worried about Miyu, she's very obedient."
Jiang You's resolute tone left Oath Emiya flustered. No matter how well-behaved Miyu was, she was still only ten years old and lacked understanding of the world. Expecting her to care for herself alone was nothing more than a pipe dream.
"A man shouldn't shirk his responsibilities! Raise your own sister yourself!"
Jiang You extended his hand to Oath Emiya, offering an invitation.
"Come to my side! Entrust me with your Card, and I'll bring you to this Worldline too!"
Fuyuki of the Fifth Holy Grail War was an ideal place—any servant could be brought here.
Jiang You was halfway through his heroic act when he suddenly realized he couldn't obtain Oath Emiya's Card this way. A pop-up window labeled "Main Quest in Progress" blocked the contract process. It seemed summoning Oath Emiya directly would have to wait until after Julian's defeat, though by then it would likely be Darius.
"It's alright, we still have Plan B."
Jiang You decided to confront the real Julian offline. "My plan is simple: I'll come to your side and help you directly."
"Thank you for your kindness. Although I nearly defeated Angelica with Miyu's support, Julian still has at least one powerful attendant. Even Julian himself possesses considerable combat strength."
Jiang You's true abilities remained indiscernible from his appearance and aura. Only his Noble Phantasms hinted at his ability to rewrite certain rules.
"I'm not important. Miyu's safety is what matters most. As long as she can grow up happy, I'll be at peace."
"Brother..."
Tears welled in Miyu's eyes, threatening to turn the atmosphere into a melodramatic family tragedy.
"Save the sentimentality for later, you two. Oath Emiya, you wouldn't actually be happy without Miyu, would you?"
Jiang You cut off the brooding mood. "As for my strength, you don't need to worry. Even if you don't trust me, you still have her!"
Jiang You grabbed Tiamat and hoisted her high in front of him.
"Look! It's Tiamat!"
(???) Tiamat dutifully raised her hands, letting out a ferocious-sounding roar: "Awooo!"
"Tiamat is...?"
This Oath Emiya's knowledge was even more dismal. Since being adopted by Kiritsugu, he had remained inseparable from him, focusing solely on combat-related studies. As for cultural education? He was barely at an elementary school dropout level, though the Oath Emiya had recently begun attending high school like a normal student.
"Well, it seems no matter which Worldline it is, Shirou Emiya is always at risk of failing a grade."
This likely explained why there wasn't a single Shirou who had ever become a regular university student across so many Worldlines.
Shirou wanted to cry, but no tears came. Truth be told, he had never even considered the idea of attending university.
"Speaking of which, why is Miss Illya in my Deck?"
Miyu gazed at the Servant Card in her hand, utterly puzzled.
Jiang You's eyes lit up. Ever since Prisma Illya's Special Summon from Illya's Deck during their previous encounter, he had been trying to summon her again, but all attempts had failed. Now, he wouldn't let this opportunity slip by. "Summon her too! She's the friend your brother wished to meet—someone he could laugh and share joy with."