01. Masters

Yes, you can play the Master for your Servant. In fact, it's probably the safest option to avoid losing your Master and makes playing a Berserker much easier.

Masters are a vital part of the Grail War. They are the anchor that binds the Servant to the world. While the Holy Grail provides the mana needed, the Servant needs an anchor to stay tethered, which is where the Master comes in.

Anyone can be a Master. Anyone. It can be one of the founding family members. It can be an exceptionally talented Magus. It can be someone from the Church. It can be a literal nobody who works a 9 to 5 job and has absolutely zero magic talent.

One is officially a Master when they find, on the back of one of their hands, the three Command Seals. These seals are the sign that one has been chosen by the Grail to partake in the war. What these Command Seals look like are up to you, but they're red.

Masters are used to provide mana to their Servant, and their capabilities as a Master also alter a Servant's parameters (STR/AGI/END/MAN/LUK) and even Class Skills. The better the Master, the better the Servant's base Parameters and Class Skills.

Masters normally can't fight Servants on even ground. Shirou was a one in a million event, and by his own admission, he can only beat Gilgamesh with his own power. Rin clobbering Medea was a result of her playing extremely dirty and burning through years worth of jewel craft in seconds, just to match Medea's spells.

When a Master is defeated, that is, their Servant has died, they can opt to be taken under Ruler's protection under an agreement that they will not rejoin the war. If they violate this, then Ruler won't help. But so long as they are under this agreement, targeting a defeated Master will result in immediate retaliation from Ruler. As long as a Master under Ruler's protection does not directly interfere (watching is fine), then Ruler will continue to protect them.