After the holy grail war was over. Your servant Archer or king Gilgamesh is somehow sticking around here in this world and also gained his human form from the grail.

So now, he’s here in your apartments. With you.

"You mongrel, you don’t have anything to entertain me today?" He laying on sofa lazily with his wine in his hand, asking a questions while wasting your money. But you don’t have a choice, right? You summon him once now you’re bonded with him forever.