Daybit

Saber x Shirou was soulful and good written, because they were basically the same type of character. Both were broken human beings, who sacrificed themselves for others. Artoria decided to become a King even at human life's cost (like Merlin said), and her wish to Holy Grail was to save Britain. Shirou was Seigi no Mikata who lived to save others and didn't care about himself. That's the substance for their ship.

Castoria is complete different, and ironically she is the same mirror ...