Quiet. Sultry. Deadly.
Rider is seemingly a very weak servant, but this is due to her master Shinji having no power. To compensate, he sets up boundary fields to take people's energy to sustain her. Shinji controls her by a command spell bound to a book he carries, not by a physical spell binding the other masters have. Even so, Rider is utterly obedient. Shinji views his servant as a tool to be used and nothing more. He loathes her lack of power, but doesn't seem to understand it's because of him.
In the first two stories, Rider isn't seen that much. She's appears, she fights, people chase Shinji around and she ends up dead. In Heaven's Feel she survives through the entire story. She doesn't speak very often, but parts of her personality show through. Although seemingly sadistic, and cold hearted, she's can also be warm and strangely sentimental. She's self concious about being tall, believing it's not very elegant.[ + ] Show Spoiler:
Shinji is actually not Rider's master. Her true master is Sakura Matou: Shinji's sister. Sakura didn't want to partake in the grail war, so one of her command spells were transferred to a book so Shinji could control her. While Rider obeys his commands, she doesn't seem to have any true loyalty towards him.
It is said a master summons a particular servant because the two have similar souls (or another common link). While appearing to be opposites, they have deeper similarities. Sakura's inner termoil is much like Rider's outward heartlessness. While it only shows when she lets her guard down, Rider's thoughtfulness is like Sakura's kind heart. This similarity gives Rider a much deeper loyalty to Sakura beyond the command spell. Rider tells Shiro that she and Sakura rarely speak to other because they don't have to: they already understand one another.
Rider's main combat ability stems from pikes she carries. The chains connected to them are a bigger threat than the pikes themselves. Once the pike strikes an opponent, that spot is bound by the chain even if the pike is removed. In her past life Rider was Medusa, giving her the power of a freezing gaze - an ability she almost never uses. This would give away her identity as a servant, which is why she wears a visor. It's also most effective when not expected. As class rider, she is capable of summoning beasts of legend, although Pegasus is the only one summoned in the story.
Is it that unexpected for me to have such an emotion?