Saber is Shirou Emiya's heroic servant during the 4th Holy Grail War and is the Saber Class servant. She is in actuality, King Arthur. Her personality is very serious, loyal, independent, and a little reserved. Saber is very strong and is often underpowered by Shirou's lack of mana, which leads to complicated relationship.

In Carnival Phantasm, Saber's personality is still very serious, but her over-seriousness is often played for laughs, as she's inadvertently being funny. She also shows a cuter, softer side in Carnival Phantasm, especially if it involves Shirou and making her food.

I think Saber should have a medium to medium low voice, serious in nature, but with the range an over the top show can have. She also turns into her Alter form, evil form, in Carnival Phantasm briefly, and she should be voiced a little more sinister in tone in that scene.  Feel free to come up with your own approach!