Who is Cage?

Simply:

Cage is the sniper for the squad, Onora, as such his skill is pretty impressive.

Cage is, however, more robotic than the rest of the squad. He isn't one to show many emotions to anyone, which is explained by his back story.

He is the younger brother of the lone spartan: "Ulysses". Being brought up to follow in Ulysses' footsteps, Cage felt that he needed a break, and while he serves the UNSC. He likes to be a sniper, sitting at the back of the conflict, taking a shot to save others without having to get too involved.

It allows him to think. Weirdly being relaxing for him.