yes it is a spartan unlockable i believe as someone said before it is Nicole, but her spartan number is hard to remember something line Spartan-458, but yeah you unlock her by completing the entire arcade campaign with every character, then you get nicole and a special arena Passat Station or something like that, there are different unlockable costumes for here, like spartan blue-red-and white (u automatically get green), i dont know if there are any others i only have those four,
when fighting with her she does have a kick a&& grenade stick, she pulls the person in the air-activates the plasma grenade-punches the person and in doing so she sticks the person, tosses the person in the air a bit then kicks them, and after a few seconds of recovery the plasma blows up,
her taunt is actually going invisible, its pretty cool, its one of the coolest characters, as bungie looked at the game, they said it was exactly how they imagined a spartan fighting in close combat with just their hands, SPARTANS destroyers of all how are in their way.