The Kiritsugu from Miyu's world died while she and Shirou were still children. He was known as the Magus Killer. After saving Shirou and adopting him, he taught him how to properly use Magecraft and took him as his assistant. Later on, they found in Fuyuki, Miyu Sakatsuki, the hidden descendant of the Child of God, making Kiritsugu take her with them to use her as a tool in order to save the world from its eventual destruction. Being caught by an unidentified disease, he has his last moments with Shirou looking at the night stars outside his garden, talking with him about his mistakes he made during his life and looking at the night sky. It is unknown if he has any relation with the Einzbern family or the 4th Ainsworth Holy Grail War.
In terms of personality, this version of Kiritsugu is most likely identic to his Fate/Zero counterpart but contrary to him, he took Shirou as his assistant and apprentice, teaching him magic instead of trying to raise him as a normal child. Wanting to save the world at all costs, he pushed himself too hard on his research and he neglected Miyu's education in order to attain his goal of using her power before it fades away, even overreacting at the fact that Shirou mentioned that they should have a birthday party to celebrate Miyu's birthday.
Fate/kaleid liner PRISMA☆ILLYA Edit
He is first mentionned by Miyu in Fate/kaleid liner PRISMA☆ILLYA 2wei! when she was telling Illya about her family, referring to him as her father. In the Fate/kaleid liner PRISMA☆ILLYA 3rei!! manga and Fate/kaleid liner PRISMA☆ILLYA Oath of Snow movie, more details about him are revealed through Shirou and Miyu's past. It showcases him saving Shirou then adopting him and going to Fuyuki city with him to get more informations about the Sakatsuki family. He dies from an unknown disease, in a similar way to his Fate/Zero and Fate/stay night counterparts.
Later on in the present, Shirou gives to Chloe von Einzbern Kiritsugu's Thompson Contender with his Origin Bullets, telling her that he wanted to entrust her with it because of her way of thinking being similar to his. She then describes the bullet as an emotionless memento, just like Kiritsugu, recognising it by saying that it's related to his other name, calling it the magic bullets of the Magus Killer.