So we have Ruby Sword, Royal Bodyguard, Military Advisor and General to Princess EastStern. He’s a little older than Human Ruby, in his mid-20’s rather than late teens, and is even more stern and strict than his human counterpart, due to his role (and lack of Sunset in his life).
He has a sword with a crystalline blade that can cut through and disrupt magic, allowing him to protect his troops and princess from most types of magic. A sufficiently powerful unicorn can crack the blade, and a Princess could shatter it entirely without too much trouble, but the blade will eventually reform. The blade glows when magic is actively used around it, allowing him to detect when someone is using magic nearby, and it will glow when a Changeling changes, but not around a disguised Changeling, only when the magic is active. It can also glow and detect Discord, but can’t affect his chaos magic.
Ruby Sword is a strict, no-nonsense warrior who seems to always be on the job and guarded, even when off duty, and is very dry and snarky when he does speak (and isn’t barking orders). He’s quite antisocial and solitary, but despite his stern red eyes and intense glare sending ponies scurrying, and his overprotectiveness of his princess sometimes, he’s quite calm and mature, not quick to anger despite his strictness, and can actually be quite laid back when not on duty. And thanks to his sweet tooth he’s even good friends with Pinkie Pie, and doesn’t find her annoying, enjoying and admirning her dedication to her friends and making others smile. (And he even shares a love of terrible puns with her)