Zelretch was bored.
Kischur Zelretch Schweinorg, known by the names of 'The Wizard Marshall,' 'Zelretch of the Jewels,' 'Old Man of the Jewels' and 'Kaleidoscope,' was bored out of his mind.
When any Magi, those cruel and heartless users of mystic circuits, hear those words, they, without exception, panic like headless chickens and attempt to flee the area. They are not alone in this. The Church, True Ancestors, Dead Apostles and other supernatural beings all instinctively know that if the Kaleidoscope is bored, things (mostly people) tend to get broken in the process of him attempting to alleviate said boredom.
The Second Magic, known as 'Operation of Parallel Worlds,' was the perfect tool to aid in the alleviation of boredom. A little known fact was that although parallel worlds were metaphorically right next to the original world, their time streams were not. Meaning that, with a bit of finagling, he could enter into a world at any point before the moment he had left his own world.
Currently, the bored Dead Apostle Ancestor was strolling along the Clocktower's corridors after witnessing something amazing, even to him; Rin Tohsaka , or to use her own country's form Tōsaka Rin, standing alongside a young man with determined green eyes and red hair, as well as a young homunculus, wielding his own Jewelled Sword Zelretch!
He had given the blueprints of his blade to the then-Head of the Tōsaka Clan and student of his, Tōsaka Nagato, two-hundred years ago as 'a bit of light homework,' when in reality, the creation of the blade would take a millennia of careful research and development to achieve, so his surprise at seeing a copy of it in existence a full eight-hundred years early was genuine.
Upon hearing that it was thanks to the boy, Emiya Shirou, and the girl, Illyasviel 'Illya' von Einzbern, that she had been able to create a copy of the Jewelled Sword, he guffawed loudly. Never had he suspected that a Magus, especially a Magus of the overly proud and independent lineage of Tōsaka Nagato, would actually enlist others in their research!
Seeing the boy Emiya, he saw at once how he had been able to help the Tōsaka girl; he had a Reality Marble, a Koyū Kekkai, as well as a completely nonstandard Origin and Element, both of them [Sword]. He thought back, but could not recall anyone having such an alignment. The closest was someone having the Origin [Pierce], which allowed him to pierce through barriers and lies easily. The downside of that one was that the holder in question literally couldn't stop himself from trying to discover the truth or breaking through barriers, which had led to a gristly end for the man.
The girl was the Einzbern's [Lesser Grail], one that had survived the Holy Grail War, but was living on borrowed time. She referred to Emiya as 'Onii-chan' although she was at least six years his senior, despite her little-girl appearance. When he discovered that Emiya Kiritsugu was her blood father and the boy's adopted father, he had laughed once again.
The inclusion of the girl explained how the boy had been able to copy, even superficially, his Jewelled Sword. He had used Illya's inherited memories of her ancestress, Justeaze Lizrich von Einzbern, to create the sword via his unique application of Graduation Air.
With his abilities at her command, Tōsaka Rin would be able to create a copy of the Jewelled Sword in decades, three at the earliest. As a reward for amusing him, he would stop the Clocktower from charging them with anything for their actions during the Fifth Holy Grail War.
He had listened to their tales of what had become of the Grail and had shaken his head at the stupidity of the Einzbern Family and their single-minded pursuit of their lost Third Magic. He would have to have a word with the current head of the family, as well as Makiri Zouken. Zelretch enjoyed surprising and interesting events, but the appearance of [Angra Mainyu: All the World's Evil] in the real world was a threat as big as the Crimson Moon had been and he would not permit it to actualise. The Grail would be disassembled.
The Church would also have to be dealt with. Kotomine Kirei had been actively engaged in trying to summon Angra Mainyu, despite his post as the so-called 'impartial' observer of the Holy Grail War. He had killed Bazett Fraga McRemitz to steal her Servant, Lancer, and had almost succeeded in bringing forth Angra Mainyu had it not been for the actions of Emiya Shirou and Tōsaka Rin.
A cursory glance at other parallel worlds indicated that the Fifth Holy Grail War was a hotbed of trouble, but that the day was almost always saved by Emiya Shirou, Tōsaka Rin and a varied mix of other individuals. Only a handful of times had things escalated to the point of a Counterforce Intervention, and those had been worlds where Emiya Shirou was either dead or not a Magus.
'A single boy being such a vital keystone to such a disastrous event…how interesting.' Zelretch thought in amusement as he breezed through the Clocktower, arranging for the charges to be dropped and laying the groundwork for the disassembly of the Greater Grail, 'Not only that, the boy seems to have the Shōnen Protagonist ability to draw women to him like bees to honey.'
Indeed, in almost every world, Emiya Shirou was surrounded by women; his guardian Fujimura Taiga, Tōsaka Rin and her estranged sister Matō/Makiri or Tōsaka Sakura, Artoria Pendragon, Medusa, Medea, Luviagelita Edelfelt and others being prominent characters in his life. Oftentimes, he fell for one in particular, but the boy sometimes had both of the estranged Tōsaka sisters and sometimes his former Servant, Saber, was his lover along with Tōsaka Rin, maintained in this world by a series of frequent tantric rituals.
With such interesting people, there was no way he was not going to interfere with them, was there?