saber limitations

The cause was Saber. Despite being a Heroic Spirit, she had limits other Servants didn't have. The gravest amongst these was that she could not dematerialize. She didn't have the ability to cancel her physical form to move at high speed or cut down the prana consumption from her Master when at rest, something all Servants should be able to do. It's not that there had been a mistake in Kiritsugu's contract or his summoning: the soul of the hero named Artoria functioned differently from the other Heroic Spirits... Apparently. For a reason unknown to even Irisviel.

The most problematic burden was that she was unable to turn invisible and hide her existence to the other people. There was no way she would walk under her armor suit, so Saber had to dress up as a human with a costume of the latest fashion, and accompany Irisviel.

- Fate/Zero: Act 3, Part 2