diff --git a/entity/item.go b/entity/item.go
new file mode 100644
index 0000000..837a5a1
--- /dev/null
+++ b/entity/item.go
@@ -0,0 +1,36 @@
+package entity
+
+type ItemType int
+
+const (
+	ItemWeapon ItemType = iota
+	ItemArmor
+	ItemConsumable
+)
+
+type Item struct {
+	Name  string
+	Type  ItemType
+	Bonus int
+	Price int
+}
+
+type RelicEffect int
+
+const (
+	RelicHealOnKill RelicEffect = iota
+	RelicATKBoost
+	RelicDEFBoost
+	RelicGoldBoost
+)
+
+type Relic struct {
+	Name   string
+	Effect RelicEffect
+	Value  int
+	Price  int
+}
+
+func NewHPPotion() Item {
+	return Item{Name: "HP Potion", Type: ItemConsumable, Bonus: 30, Price: 20}
+}
diff --git a/entity/monster.go b/entity/monster.go
new file mode 100644
index 0000000..85e0e48
--- /dev/null
+++ b/entity/monster.go
@@ -0,0 +1,83 @@
+package entity
+
+import "math"
+
+type MonsterType int
+
+const (
+	MonsterSlime MonsterType = iota
+	MonsterSkeleton
+	MonsterOrc
+	MonsterDarkKnight
+	MonsterBoss5
+	MonsterBoss10
+	MonsterBoss15
+	MonsterBoss20
+)
+
+type monsterBase struct {
+	Name     string
+	HP, ATK, DEF int
+	MinFloor int
+	IsBoss   bool
+}
+
+var monsterDefs = map[MonsterType]monsterBase{
+	MonsterSlime:      {"Slime", 20, 5, 1, 1, false},
+	MonsterSkeleton:   {"Skeleton", 35, 10, 4, 3, false},
+	MonsterOrc:        {"Orc", 55, 14, 6, 6, false},
+	MonsterDarkKnight: {"Dark Knight", 80, 18, 10, 12, false},
+	MonsterBoss5:      {"Guardian", 150, 15, 8, 5, true},
+	MonsterBoss10:     {"Warden", 250, 22, 12, 10, true},
+	MonsterBoss15:     {"Overlord", 400, 30, 16, 15, true},
+	MonsterBoss20:     {"Archlich", 600, 40, 20, 20, true},
+}
+
+type Monster struct {
+	Name        string
+	Type        MonsterType
+	HP, MaxHP   int
+	ATK, DEF    int
+	IsBoss      bool
+	TauntTarget bool
+	TauntTurns  int
+}
+
+func NewMonster(mt MonsterType, floor int) *Monster {
+	base := monsterDefs[mt]
+	scale := 1.0
+	if !base.IsBoss && floor > base.MinFloor {
+		scale = math.Pow(1.15, float64(floor-base.MinFloor))
+	}
+	hp := int(math.Round(float64(base.HP) * scale))
+	atk := int(math.Round(float64(base.ATK) * scale))
+	return &Monster{
+		Name:   base.Name,
+		Type:   mt,
+		HP:     hp,
+		MaxHP:  hp,
+		ATK:    atk,
+		DEF:    base.DEF,
+		IsBoss: base.IsBoss,
+	}
+}
+
+func (m *Monster) TakeDamage(dmg int) {
+	m.HP -= dmg
+	if m.HP < 0 {
+		m.HP = 0
+	}
+}
+
+func (m *Monster) IsDead() bool {
+	return m.HP <= 0
+}
+
+func (m *Monster) TickTaunt() {
+	if m.TauntTurns > 0 {
+		m.TauntTurns--
+		if m.TauntTurns == 0 {
+			m.TauntTarget = false
+		}
+	}
+}
diff --git a/entity/monster_test.go b/entity/monster_test.go
new file mode 100644
index 0000000..a230e15
--- /dev/null
+++ b/entity/monster_test.go
@@ -0,0 +1,25 @@
+package entity
+
+import (
+	"testing"
+	"math"
+)
+
+func TestMonsterScaling(t *testing.T) {
+	slime := NewMonster(MonsterSlime, 1)
+	if slime.HP != 20 || slime.ATK != 5 {
+		t.Errorf("Slime floor 1: got HP=%d ATK=%d, want HP=20 ATK=5", slime.HP, slime.ATK)
+	}
+	slimeF3 := NewMonster(MonsterSlime, 3)
+	expectedHP := int(math.Round(20 * math.Pow(1.15, 2)))
+	if slimeF3.HP != expectedHP {
+		t.Errorf("Slime floor 3: got HP=%d, want %d", slimeF3.HP, expectedHP)
+	}
+}
+
+func TestBossStats(t *testing.T) {
+	boss := NewMonster(MonsterBoss5, 5)
+	if boss.HP != 150 || boss.ATK != 15 || boss.DEF != 8 {
+		t.Errorf("Boss5: got HP=%d ATK=%d DEF=%d, want 150/15/8", boss.HP, boss.ATK, boss.DEF)
+	}
+}
diff --git a/entity/player.go b/entity/player.go
new file mode 100644
index 0000000..ccb2628
--- /dev/null
+++ b/entity/player.go
@@ -0,0 +1,96 @@
+package entity
+
+type Class int
+
+const (
+	ClassWarrior Class = iota
+	ClassMage
+	ClassHealer
+	ClassRogue
+)
+
+func (c Class) String() string {
+	return [...]string{"Warrior", "Mage", "Healer", "Rogue"}[c]
+}
+
+type classStats struct {
+	HP, ATK, DEF int
+}
+
+var classBaseStats = map[Class]classStats{
+	ClassWarrior: {120, 12, 8},
+	ClassMage:    {70, 20, 3},
+	ClassHealer:  {90, 8, 5},
+	ClassRogue:   {85, 15, 4},
+}
+
+type Player struct {
+	Name        string
+	Fingerprint string
+	Class       Class
+	HP, MaxHP   int
+	ATK, DEF    int
+	Gold        int
+	Inventory   []Item
+	Relics      []Relic
+	Dead        bool
+}
+
+func NewPlayer(name string, class Class) *Player {
+	stats := classBaseStats[class]
+	return &Player{
+		Name:  name,
+		Class: class,
+		HP:    stats.HP,
+		MaxHP: stats.HP,
+		ATK:   stats.ATK,
+		DEF:   stats.DEF,
+	}
+}
+
+func (p *Player) TakeDamage(dmg int) {
+	p.HP -= dmg
+	if p.HP <= 0 {
+		p.HP = 0
+		p.Dead = true
+	}
+}
+
+func (p *Player) Heal(amount int) {
+	p.HP += amount
+	if p.HP > p.MaxHP {
+		p.HP = p.MaxHP
+	}
+}
+
+func (p *Player) IsDead() bool {
+	return p.Dead
+}
+
+func (p *Player) Revive(hpPercent float64) {
+	p.Dead = false
+	p.HP = int(float64(p.MaxHP) * hpPercent)
+	if p.HP < 1 {
+		p.HP = 1
+	}
+}
+
+func (p *Player) EffectiveATK() int {
+	atk := p.ATK
+	for _, item := range p.Inventory {
+		if item.Type == ItemWeapon {
+			atk += item.Bonus
+		}
+	}
+	return atk
+}
+
+func (p *Player) EffectiveDEF() int {
+	def := p.DEF
+	for _, item := range p.Inventory {
+		if item.Type == ItemArmor {
+			def += item.Bonus
+		}
+	}
+	return def
+}
diff --git a/entity/player_test.go b/entity/player_test.go
new file mode 100644
index 0000000..7b87460
--- /dev/null
+++ b/entity/player_test.go
@@ -0,0 +1,53 @@
+package entity
+
+import "testing"
+
+func TestNewPlayer(t *testing.T) {
+	p := NewPlayer("testuser", ClassWarrior)
+	if p.HP != 120 || p.MaxHP != 120 {
+		t.Errorf("Warrior HP: got %d, want 120", p.HP)
+	}
+	if p.ATK != 12 {
+		t.Errorf("Warrior ATK: got %d, want 12", p.ATK)
+	}
+	if p.DEF != 8 {
+		t.Errorf("Warrior DEF: got %d, want 8", p.DEF)
+	}
+	if p.Gold != 0 {
+		t.Errorf("Initial gold: got %d, want 0", p.Gold)
+	}
+}
+
+func TestAllClasses(t *testing.T) {
+	tests := []struct {
+		class    Class
+		hp, atk, def int
+	}{
+		{ClassWarrior, 120, 12, 8},
+		{ClassMage, 70, 20, 3},
+		{ClassHealer, 90, 8, 5},
+		{ClassRogue, 85, 15, 4},
+	}
+	for _, tt := range tests {
+		p := NewPlayer("test", tt.class)
+		if p.HP != tt.hp || p.ATK != tt.atk || p.DEF != tt.def {
+			t.Errorf("Class %v: got HP=%d ATK=%d DEF=%d, want HP=%d ATK=%d DEF=%d",
+				tt.class, p.HP, p.ATK, p.DEF, tt.hp, tt.atk, tt.def)
+		}
+	}
+}
+
+func TestPlayerTakeDamage(t *testing.T) {
+	p := NewPlayer("test", ClassWarrior)
+	p.TakeDamage(30)
+	if p.HP != 90 {
+		t.Errorf("HP after 30 dmg: got %d, want 90", p.HP)
+	}
+	p.TakeDamage(200)
+	if p.HP != 0 {
+		t.Errorf("HP should not go below 0: got %d", p.HP)
+	}
+	if !p.IsDead() {
+		t.Error("Player should be dead")
+	}
+}