diff --git a/application/index.ts b/application/index.ts
index 36c0d44..e63fb96 100644
--- a/application/index.ts
+++ b/application/index.ts
@@ -1 +1 @@
-export * as quest from "application/Quest";
+export * from "./Quest";
diff --git a/core/Account/Avatar.ts b/core/Account/Avatar.ts
new file mode 100644
index 0000000..1e0c453
--- /dev/null
+++ b/core/Account/Avatar.ts
@@ -0,0 +1,23 @@
+import { Item } from "core";
+
+export enum Race {
+  Egg,
+  Plant,
+}
+
+export class Avatar {
+  /**
+   * Level function is `50x ^ 2`.
+   */
+  public get level(): number {
+    return this.totalExp == 0 ? 0 : Math.sqrt(this.totalExp / 50);
+  }
+
+  constructor(public race: Race, public totalExp: number) {}
+
+  public applyItem(...items: Item[]): void {
+    items.forEach((item) => {
+      item.takeEffect();
+    });
+  }
+}
diff --git a/core/Account/User.test.ts b/core/Account/User.test.ts
new file mode 100644
index 0000000..03aa257
--- /dev/null
+++ b/core/Account/User.test.ts
@@ -0,0 +1,32 @@
+import { IAccountRepository } from "infrastructure";
+import { Avatar, Race } from "./Avatar";
+import { Player, User } from "./User";
+
+describe("User's functionality", () => {
+  test("password hashing", () => {
+    const cases = [
+      { raw: "password", hash: "5e884898da28047151d0e56f8dc6292773603d0d6aabbdd62a11ef721d1542d8" },
+      { raw: "cello", hash: "9bbf02efd82322aadc5d06c9bcf35bb4b0e3302ca158dc800407be1a4fea67e2" },
+    ];
+
+    cases.forEach((c) => {
+      expect(User.hashPassword(c.raw)).toBe(c.hash.toLowerCase());
+    });
+  });
+});
+
+describe("Player's functionality", () => {
+  test("player level", () => {
+    [
+      { exp: 0, lv: 0 },
+      { exp: 1250, lv: 5 },
+      { exp: 5000, lv: 10 },
+    ].forEach(async (c) => {
+      const repoMock = jest.fn<Avatar, []>(() => new Avatar(Race.Egg, c.exp));
+      const p = await Player.New({ getAvatar: repoMock } as unknown as IAccountRepository, "", "");
+      const lv = p.level;
+      expect(repoMock).toBeCalledTimes(1);
+      expect(lv).toBe(c.lv);
+    });
+  });
+});
diff --git a/core/Account/User.ts b/core/Account/User.ts
new file mode 100644
index 0000000..f961fc6
--- /dev/null
+++ b/core/Account/User.ts
@@ -0,0 +1,81 @@
+import { Avatar } from "core";
+import crypto from "crypto";
+import { IAccountRepository } from "infrastructure";
+
+export abstract class User {
+  public password?: string;
+
+  constructor(public repo: IAccountRepository, public accountId: string, public email: string) {}
+
+  public async login(options: LoginOptions): Promise<boolean> {
+    if (this.password === undefined) {
+      this.password = await this.repo.getUserPassword(this.accountId);
+    }
+
+    let result: boolean = this.password === User.hashPassword(options.password);
+    let timestamp: number = Date.now();
+
+    if (result) {
+      this.repo.setLastLogin(timestamp);
+    } else {
+      this.repo.setLastLoginAttempt(timestamp);
+    }
+
+    return result;
+  }
+
+  public static hashPassword(password: string): string {
+    return crypto.createHash("sha256", {}).update(password).digest("hex");
+  }
+
+  public static register(repo: IAccountRepository, email: string, password: string): Promise<void> {
+    password = this.hashPassword(password);
+    return Promise.resolve(
+      repo.registerNewUser({ repo: repo, accountId: "", email: email, password: password } as User)
+    );
+  }
+
+  public async updatePassword(password: string): Promise<boolean> {
+    this.password = this.password ?? (await this.repo.getUserPassword(this.accountId));
+
+    password = User.hashPassword(password);
+
+    if (password === this.password) {
+      return false;
+    }
+
+    this.repo.updateUserPassword(this.accountId, password);
+    return true;
+  }
+
+  public upgradeToPlayer(avatar: Avatar): Player {
+    const p = new Player(this.repo, this.accountId, this.email);
+    p.avatar = avatar;
+    return p;
+  }
+}
+
+export class LoginOptions {
+  constructor(public email: string, public password: string) {}
+}
+
+export class Player extends User {
+  public avatar!: Avatar;
+
+  public get level(): number {
+    return this.avatar.level;
+  }
+
+  /**
+   * プレイヤーをインスタンス化する場合は`New`を使ってください。
+   */
+  constructor(repo: IAccountRepository, accountId: string, email: string) {
+    super(repo, accountId, email);
+  }
+
+  public static async New(repo: IAccountRepository, accountId: string, email: string): Promise<Player> {
+    const p = new Player(repo, accountId, email);
+    p.avatar = await p.repo.getAvatar(p.accountId);
+    return p;
+  }
+}
diff --git a/domain/Quest/Answer.ts b/core/Quest/Answer.ts
similarity index 100%
rename from domain/Quest/Answer.ts
rename to core/Quest/Answer.ts
diff --git a/domain/Quest/Effect.ts b/core/Quest/Effect.ts
similarity index 100%
rename from domain/Quest/Effect.ts
rename to core/Quest/Effect.ts
diff --git a/domain/Quest/Item.ts b/core/Quest/Item.ts
similarity index 85%
rename from domain/Quest/Item.ts
rename to core/Quest/Item.ts
index 88d3ca8..acaf87a 100644
--- a/domain/Quest/Item.ts
+++ b/core/Quest/Item.ts
@@ -1,4 +1,4 @@
-import { Effect, EffectOption } from "domain/Quest";
+import { Effect, EffectOption } from "core";
 
 export abstract class Item {
   public effect: Effect | null;
diff --git a/domain/Quest/Quest.ts b/core/Quest/Quest.ts
similarity index 91%
rename from domain/Quest/Quest.ts
rename to core/Quest/Quest.ts
index 9a16549..991002f 100644
--- a/domain/Quest/Quest.ts
+++ b/core/Quest/Quest.ts
@@ -1,4 +1,4 @@
-import { Answer, Item } from "domain/Quest";
+import { Answer, Item } from "core";
 
 export abstract class Quest {
   private solution: Answer;
diff --git a/core/index.ts b/core/index.ts
new file mode 100644
index 0000000..feec093
--- /dev/null
+++ b/core/index.ts
@@ -0,0 +1,6 @@
+export * from "./Account/Avatar";
+export * from "./Account/User";
+export * from "./Quest/Answer";
+export * from "./Quest/Effect";
+export * from "./Quest/Item";
+export * from "./Quest/Quest";
diff --git a/domain/Account/Avatar.ts b/domain/Account/Avatar.ts
deleted file mode 100644
index 8f61de1..0000000
--- a/domain/Account/Avatar.ts
+++ /dev/null
@@ -1,11 +0,0 @@
-import { Player } from "domain/Account";
-import { IAccountRepository } from "infrastructure";
-
-export enum Race {
-  Egg,
-  Plant,
-}
-
-export class Avatar {
-  constructor(public repo: IAccountRepository, public player: Player, public race: Race) {}
-}
diff --git a/domain/Account/User.ts b/domain/Account/User.ts
deleted file mode 100644
index 2896829..0000000
--- a/domain/Account/User.ts
+++ /dev/null
@@ -1,41 +0,0 @@
-import { Avatar } from "domain/Account";
-import { IAccountRepository } from "infrastructure";
-
-export abstract class User {
-  constructor(
-    public repo: IAccountRepository,
-    public accountId: string,
-    public email: string,
-    public password: string
-  ) {}
-
-  public login(options: LoginOptions): boolean {
-    let result: boolean = this.password !== options.password;
-    let timestamp: number = Date.now();
-
-    if (result) {
-      this.repo.setLastLoginAttempt(timestamp);
-    } else {
-      this.repo.setLastLogin(timestamp);
-    }
-
-    return result;
-  }
-}
-
-export class LoginOptions {
-  constructor(public email: string, public password: string) {}
-}
-
-export class Player extends User {
-  constructor(
-    repo: IAccountRepository,
-    accountId: string,
-    email: string,
-    password: string,
-    public totalExp: number,
-    public avatar: Avatar
-  ) {
-    super(repo, accountId, email, password);
-  }
-}
diff --git a/domain/Account/index.ts b/domain/Account/index.ts
deleted file mode 100644
index 4a5d804..0000000
--- a/domain/Account/index.ts
+++ /dev/null
@@ -1,2 +0,0 @@
-export * from "./Avatar";
-export * from "./User";
diff --git a/domain/Quest/index.ts b/domain/Quest/index.ts
deleted file mode 100644
index 70224a9..0000000
--- a/domain/Quest/index.ts
+++ /dev/null
@@ -1,4 +0,0 @@
-export * from "./Answer";
-export * from "./Effect";
-export * from "./Item";
-export * from "./Quest";
diff --git a/domain/index.ts b/domain/index.ts
deleted file mode 100644
index 403cf0e..0000000
--- a/domain/index.ts
+++ /dev/null
@@ -1,2 +0,0 @@
-export * as account from "domain/Account";
-export * as quest from "domain/Quest";
diff --git a/index.ts b/index.ts
index 5a9989f..fc80201 100644
--- a/index.ts
+++ b/index.ts
@@ -1,3 +1,3 @@
-export * as domain from "./domain";
+export * as domain from "./core";
 export * as infrastructure from "./infrastructure";
 export * as application from "./application";
diff --git a/infrastructure/index.ts b/infrastructure/index.ts
index 00a1869..2c8d38c 100644
--- a/infrastructure/index.ts
+++ b/infrastructure/index.ts
@@ -1,22 +1,32 @@
-import { Avatar, Player, User } from "domain/Account";
-import { Answer, Item, Quest } from "domain/Quest";
+import { Answer, Avatar, Item, Player, Quest, User } from "core";
 
 export type Identifier = string | number;
 
 export interface IQuestRepository {
-  getQuest: (id: Identifier) => Quest;
-  getAnswer: (id: Identifier) => Answer;
-  getItem: (id: Identifier) => Item;
+  getQuest: (id: Identifier) => Promise<Quest>;
+  getAnswer: (id: Identifier) => Promise<Answer>;
+  getItem: (id: Identifier) => Promise<Item>;
 }
 
 export interface IAccountRepository {
-  getUser(id: Identifier): User;
-  getPlayer(id: Identifier): Player;
-  getAvatar(id: Identifier): Avatar;
+  getUser(id: Identifier): Promise<User>;
+  getUserPassword(id: Identifier): Promise<string>;
+  getPlayer(id: Identifier): Promise<Player>;
+  /**
+   * プレイヤーのIDからアバターを取得する。
+   * @param player プレイヤーのID
+   */
+  getAvatar(player: Identifier): Promise<Avatar>;
+  /**
+   * ユーザーのパスワードを更新する。
+   * @param user ユーザーのID
+   * @param password 新しいパスワードのハッシュ
+   */
+  updateUserPassword(user: Identifier, password: string): Promise<void>;
 
-  setLastLoginAttempt(timestamp: number): void;
-  setLastLogin(timestamp: number): void;
-  registerNewUser(user: User): void;
+  setLastLoginAttempt(timestamp: number): Promise<void>;
+  setLastLogin(timestamp: number): Promise<void>;
+  registerNewUser(user: User): Promise<void>;
   upgradeUserToPlayer(user: User, player: Player): Promise<void>;
   unregisterUser(id: Identifier): Promise<void>;
   unregisterPlayer(id: Identifier): Promise<void>;
diff --git a/tsconfig.json b/tsconfig.json
index 6725053..faa86cd 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -21,5 +21,5 @@
     "allowUnreachableCode": false,
     "skipLibCheck": true
   },
-  "include": ["application/**/*.ts", "domain/**/*.ts", "infrastructure/**/*.ts"]
+  "include": ["application/**/*.ts", "core/**/*.ts", "infrastructure/**/*.ts"]
 }