Ruby's job in the System (where all the Critters live) is security - her territory entirely surrounds the System's core, and her job is to keep everyone out of it. The Nerve Center, as it's called, is a dangerous place anyway, full of lasers, conveyor belts, and spike traps; it takes quite a bit of agility to make it through there intact, and even if you do, Ruby's own tail serves as a laser cannon - and there's a reason she's got a targeting sight for a symbol. Sometimes, though, things get past her, or someone manages to catch her by surprise - which is the expression I decided to put on her.
Ruby's arms are different sizes on purpose; the upper pair are smaller and more dexterous, while the lower pair are thick and muscular. She can function equally well on two, four, or six limbs, depending on how she wants to trade off speed for dexterity.