Sir Percival is a King Arthur hero and is unlocked at 70,000 prestige.
Sir Percival is a Gambit hero with above average Bravery, high Tactics, low Magic. His unit type, Pavise Crossbowmen, grants a high chance to block normal attacks. Blocked attacks do no damage and are met with a counterattack by the blocker.
His gambit, Covering Fire, Launches an attack that hits a single target and applies a defense boost to himself as well as returning his morale to 100. With a single flag bearer, Sir Percival can continuously unleash gambit attacks while tanking all forms of damage.
He works well in the Trident and Mallet formations. Trident increases all defense and Mallet increases Gambit Damage.
He is a great secondary gambit hero. Secondary meaning you should use a hero like Sir Galahad, Sir Balin or Attila as your primary attacker with Sir Percival being in more of a support role.