おお、ほんとだできた。
protocol CharacterProtocol { init(name: String, hp: Int) var name: String { get } var hp: Int { get } mutating func update(hp: Int) } extension CharacterProtocol { mutating func update(hp: Int) { self = Self.init(name: name, hp: hp) } } class Character: CharacterProtocol { let name: String let hp: Int required init(name: String, hp: Int) { self.name = name self.hp = hp } } var character = Character(name: "ゆうしゃ", hp: 153) character.update(hp: character.hp - 10)