The reader is a friend of Shirou's, and helped in destroying the grail, however after Saber destroyed it, the reader was suddenly drawn to a vortex before disappearing. He falls into a white void, where he meets Irisveil, who tells him that he has been transported to another world, one that's still in the era of the gods, as well as the fact the Grail had not been destroyed, simply turned into a energy based state rather then a material based one, and that it had been absorbed by his body. Combined with the fact that the world is continuously supplying it with magic, he can summon servants, multiple even, and with how it's still connected to his home world, Saber still has a body and is with Shirou.

The readers origin is the concept of 'The Iron Sun', the alchemic concept that the sun is made of molten Iron, because this concept was debunk/killed, it was reborn as the reader, so he can create waves and blasts of molten iron, and purify the darkness of steel, and souls.

Servants summoned:
first- Francis Drake(Rider)
Second- Nursery Rhythm(Caster)(parental/child relationship)