How does the Grail choose the participants?
- We know that the Grail tries to pick people who have some sort of strong desire (when Kotomine asks Tohsaka at the beginning of Fate/Zero).
- Do you need to descend from a mage bloodline? Did the murderer guy (the one who got Caster in Fate/Zero) have magical blood?
- It seems like you don't need to be aware of the existence of the Holy Grail war (Shirou and the murderer guy were both pretty clueless).
- You do seem to need an heroic relic... or do you? When Kotomine was surprisingly chosen in Fate/Zero, did he have one already? What if you lose the relic before the summoning occurs? (In Fate/Zero the participants were already making preparations for years before the summoning).
- The Matou were disappointed that Kariya left behind their magical teachings, so they decided to adopt Sakura to prepare her for the next war. When Kariya returns and chooses to fight, he successfully becomes a master. This suggests that there is a way to opt out/in from the war (?)
So how does the Grail pick participants exactly?