Kind of a meme. A melee unit who, unlike executioner, is actually good at it. Basically, she has infinite shield that keeps stacking as long as she attacks. Most bosses are not meant to fight this sort of thing, so she basically can destroy Yegor with no player input outside of telling her to go attack him. She isn't invincible or invulnerable, and is particularly vulnerable to burst damage enemies (Hydras come to mind, or SWAP Jaegers), but basic enemy fire wont even phase her.

Another issue is that friendly SF aren't invulnerable to stun, and some enemies can stun her. When that happens, she goes down pretty hard because she can't replenish her defenses. Otherwise, her overwhelming defensive ability lets you faceroll most encounters.