diff --git a/src/core/mod.rs b/src/core/mod.rs
index 94f1602..b76a188 100644
--- a/src/core/mod.rs
+++ b/src/core/mod.rs
@@ -2,10 +2,12 @@ pub mod analysis;
 pub mod error;
 pub mod memory;
 pub mod profile;
+pub mod relationship;
 pub mod store;
 
 pub use analysis::UserAnalysis;
 pub use error::{MemoryError, Result};
 pub use memory::Memory;
 pub use profile::{UserProfile, TraitScore};
+pub use relationship::{RelationshipInference, infer_all_relationships};
 pub use store::MemoryStore;
diff --git a/src/core/relationship.rs b/src/core/relationship.rs
new file mode 100644
index 0000000..4972be2
--- /dev/null
+++ b/src/core/relationship.rs
@@ -0,0 +1,280 @@
+use chrono::{DateTime, Utc};
+use serde::{Deserialize, Serialize};
+use std::collections::HashMap;
+
+use crate::core::{Memory, MemoryStore, UserProfile};
+use crate::core::error::Result;
+
+/// Inferred relationship with an entity (Layer 4)
+///
+/// This is not stored permanently but generated on-demand from
+/// Layer 1 memories and Layer 3.5 user profile.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct RelationshipInference {
+    /// Entity identifier
+    pub entity_id: String,
+
+    /// Total interaction count with this entity
+    pub interaction_count: u32,
+
+    /// Average priority score of memories with this entity
+    pub avg_priority: f32,
+
+    /// Days since last interaction
+    pub days_since_last: i64,
+
+    /// Inferred bond strength (0.0-1.0)
+    pub bond_strength: f32,
+
+    /// Inferred relationship type
+    pub relationship_type: String,
+
+    /// Confidence in this inference (0.0-1.0, based on data volume)
+    pub confidence: f32,
+
+    /// When this inference was generated
+    pub inferred_at: DateTime<Utc>,
+}
+
+impl RelationshipInference {
+    /// Infer relationship from memories and user profile
+    pub fn infer(
+        entity_id: String,
+        memories: &[Memory],
+        user_profile: &UserProfile,
+    ) -> Self {
+        // Filter memories related to this entity
+        let entity_memories: Vec<_> = memories
+            .iter()
+            .filter(|m| m.has_entity(&entity_id))
+            .collect();
+
+        let interaction_count = entity_memories.len() as u32;
+
+        // Calculate average priority
+        let total_priority: f32 = entity_memories
+            .iter()
+            .filter_map(|m| m.priority_score)
+            .sum();
+        let priority_count = entity_memories
+            .iter()
+            .filter(|m| m.priority_score.is_some())
+            .count() as f32;
+        let avg_priority = if priority_count > 0.0 {
+            total_priority / priority_count
+        } else {
+            0.5 // Default to neutral if no scores
+        };
+
+        // Calculate days since last interaction
+        let days_since_last = entity_memories
+            .iter()
+            .map(|m| (Utc::now() - m.created_at).num_days())
+            .min()
+            .unwrap_or(999);
+
+        // Infer bond strength based on user personality
+        let bond_strength = Self::calculate_bond_strength(
+            interaction_count,
+            avg_priority,
+            user_profile,
+        );
+
+        // Infer relationship type
+        let relationship_type = Self::infer_relationship_type(
+            interaction_count,
+            avg_priority,
+            bond_strength,
+        );
+
+        // Calculate confidence
+        let confidence = Self::calculate_confidence(interaction_count);
+
+        RelationshipInference {
+            entity_id,
+            interaction_count,
+            avg_priority,
+            days_since_last,
+            bond_strength,
+            relationship_type,
+            confidence,
+            inferred_at: Utc::now(),
+        }
+    }
+
+    /// Calculate bond strength from interaction data and user personality
+    fn calculate_bond_strength(
+        interaction_count: u32,
+        avg_priority: f32,
+        user_profile: &UserProfile,
+    ) -> f32 {
+        // Extract extraversion score (if available)
+        let extraversion = user_profile
+            .dominant_traits
+            .iter()
+            .find(|t| t.name == "extraversion")
+            .map(|t| t.score)
+            .unwrap_or(0.5);
+
+        let bond_strength = if extraversion < 0.5 {
+            // Introverted: fewer but deeper relationships
+            // Interaction count matters more
+            let count_factor = (interaction_count as f32 / 20.0).min(1.0);
+            let priority_factor = avg_priority;
+
+            // Weight: 60% count, 40% priority
+            count_factor * 0.6 + priority_factor * 0.4
+        } else {
+            // Extroverted: many relationships, quality varies
+            // Priority matters more
+            let count_factor = (interaction_count as f32 / 50.0).min(1.0);
+            let priority_factor = avg_priority;
+
+            // Weight: 40% count, 60% priority
+            count_factor * 0.4 + priority_factor * 0.6
+        };
+
+        bond_strength.clamp(0.0, 1.0)
+    }
+
+    /// Infer relationship type from metrics
+    fn infer_relationship_type(
+        interaction_count: u32,
+        avg_priority: f32,
+        bond_strength: f32,
+    ) -> String {
+        if bond_strength >= 0.8 {
+            "close_friend".to_string()
+        } else if bond_strength >= 0.6 {
+            "friend".to_string()
+        } else if bond_strength >= 0.4 {
+            if avg_priority >= 0.6 {
+                "valued_acquaintance".to_string()
+            } else {
+                "acquaintance".to_string()
+            }
+        } else if interaction_count >= 5 {
+            "regular_contact".to_string()
+        } else {
+            "distant".to_string()
+        }
+    }
+
+    /// Calculate confidence based on data volume
+    fn calculate_confidence(interaction_count: u32) -> f32 {
+        // Confidence increases with more data
+        // 1-2 interactions: low confidence (0.2-0.3)
+        // 5 interactions: medium confidence (0.5)
+        // 10+ interactions: high confidence (0.8+)
+        let confidence = match interaction_count {
+            0 => 0.0,
+            1 => 0.2,
+            2 => 0.3,
+            3 => 0.4,
+            4 => 0.45,
+            5..=9 => 0.5 + (interaction_count - 5) as f32 * 0.05,
+            _ => 0.8 + ((interaction_count - 10) as f32 * 0.02).min(0.2),
+        };
+
+        confidence.clamp(0.0, 1.0)
+    }
+}
+
+/// Generate relationship inferences for all entities in memories
+pub fn infer_all_relationships(
+    store: &MemoryStore,
+) -> Result<Vec<RelationshipInference>> {
+    // Get all memories
+    let memories = store.list()?;
+
+    // Get user profile
+    let user_profile = store.get_profile()?;
+
+    // Extract all unique entities
+    let mut entities: HashMap<String, ()> = HashMap::new();
+    for memory in &memories {
+        if let Some(ref entity_list) = memory.related_entities {
+            for entity in entity_list {
+                entities.insert(entity.clone(), ());
+            }
+        }
+    }
+
+    // Infer relationship for each entity
+    let mut relationships: Vec<_> = entities
+        .keys()
+        .map(|entity_id| {
+            RelationshipInference::infer(
+                entity_id.clone(),
+                &memories,
+                &user_profile,
+            )
+        })
+        .collect();
+
+    // Sort by bond strength (descending)
+    relationships.sort_by(|a, b| {
+        b.bond_strength
+            .partial_cmp(&a.bond_strength)
+            .unwrap_or(std::cmp::Ordering::Equal)
+    });
+
+    Ok(relationships)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::core::profile::TraitScore;
+
+    #[test]
+    fn test_confidence_calculation() {
+        assert_eq!(RelationshipInference::calculate_confidence(0), 0.0);
+        assert_eq!(RelationshipInference::calculate_confidence(1), 0.2);
+        assert_eq!(RelationshipInference::calculate_confidence(5), 0.5);
+        assert!(RelationshipInference::calculate_confidence(10) >= 0.8);
+    }
+
+    #[test]
+    fn test_relationship_type() {
+        assert_eq!(
+            RelationshipInference::infer_relationship_type(20, 0.9, 0.85),
+            "close_friend"
+        );
+        assert_eq!(
+            RelationshipInference::infer_relationship_type(10, 0.7, 0.65),
+            "friend"
+        );
+        assert_eq!(
+            RelationshipInference::infer_relationship_type(5, 0.5, 0.45),
+            "acquaintance"
+        );
+    }
+
+    #[test]
+    fn test_bond_strength_introverted() {
+        let user_profile = UserProfile {
+            dominant_traits: vec![
+                TraitScore {
+                    name: "extraversion".to_string(),
+                    score: 0.3, // Introverted
+                },
+            ],
+            core_interests: vec![],
+            core_values: vec![],
+            key_memory_ids: vec![],
+            data_quality: 1.0,
+            last_updated: Utc::now(),
+        };
+
+        // Introverted: count matters more
+        let strength = RelationshipInference::calculate_bond_strength(
+            20, // Many interactions
+            0.5, // Medium priority
+            &user_profile,
+        );
+
+        // Should be high due to high interaction count
+        assert!(strength > 0.5);
+    }
+}
diff --git a/src/mcp/base.rs b/src/mcp/base.rs
index 9e22cda..66dca78 100644
--- a/src/mcp/base.rs
+++ b/src/mcp/base.rs
@@ -2,7 +2,7 @@ use anyhow::Result;
 use serde_json::{json, Value};
 use std::io::{self, BufRead, Write};
 
-use crate::core::{Memory, MemoryStore, UserAnalysis};
+use crate::core::{Memory, MemoryStore, UserAnalysis, RelationshipInference, infer_all_relationships};
 
 pub struct BaseMCPServer {
     store: MemoryStore,
@@ -252,6 +252,33 @@ impl BaseMCPServer {
                     "properties": {}
                 }
             }),
+            json!({
+                "name": "get_relationship",
+                "description": "Get inferred relationship with a specific entity (Layer 4). Analyzes memories and user profile to infer bond strength and relationship type. Use only when game/relationship features are active.",
+                "inputSchema": {
+                    "type": "object",
+                    "properties": {
+                        "entity_id": {
+                            "type": "string",
+                            "description": "Entity identifier (e.g., 'alice', 'companion_miku')"
+                        }
+                    },
+                    "required": ["entity_id"]
+                }
+            }),
+            json!({
+                "name": "list_relationships",
+                "description": "List all inferred relationships sorted by bond strength (Layer 4). Returns relationships with all tracked entities. Use only when game/relationship features are active.",
+                "inputSchema": {
+                    "type": "object",
+                    "properties": {
+                        "limit": {
+                            "type": "number",
+                            "description": "Maximum number of relationships to return (default: 10)"
+                        }
+                    }
+                }
+            }),
         ]
     }
 
@@ -285,6 +312,8 @@ impl BaseMCPServer {
             "save_user_analysis" => self.tool_save_user_analysis(arguments),
             "get_user_analysis" => self.tool_get_user_analysis(),
             "get_profile" => self.tool_get_profile(),
+            "get_relationship" => self.tool_get_relationship(arguments),
+            "list_relationships" => self.tool_list_relationships(arguments),
             _ => json!({
                 "success": false,
                 "error": format!("Unknown tool: {}", tool_name)
@@ -518,6 +547,87 @@ impl BaseMCPServer {
         }
     }
 
+    fn tool_get_relationship(&self, arguments: &Value) -> Value {
+        let entity_id = arguments["entity_id"].as_str().unwrap_or("");
+
+        if entity_id.is_empty() {
+            return json!({
+                "success": false,
+                "error": "entity_id is required"
+            });
+        }
+
+        // Get memories and user profile
+        let memories = match self.store.list() {
+            Ok(m) => m,
+            Err(e) => return json!({
+                "success": false,
+                "error": format!("Failed to get memories: {}", e)
+            }),
+        };
+
+        let user_profile = match self.store.get_profile() {
+            Ok(p) => p,
+            Err(e) => return json!({
+                "success": false,
+                "error": format!("Failed to get profile: {}", e)
+            }),
+        };
+
+        // Infer relationship
+        let relationship = RelationshipInference::infer(
+            entity_id.to_string(),
+            &memories,
+            &user_profile,
+        );
+
+        json!({
+            "success": true,
+            "relationship": {
+                "entity_id": relationship.entity_id,
+                "interaction_count": relationship.interaction_count,
+                "avg_priority": relationship.avg_priority,
+                "days_since_last": relationship.days_since_last,
+                "bond_strength": relationship.bond_strength,
+                "relationship_type": relationship.relationship_type,
+                "confidence": relationship.confidence,
+                "inferred_at": relationship.inferred_at
+            }
+        })
+    }
+
+    fn tool_list_relationships(&self, arguments: &Value) -> Value {
+        let limit = arguments["limit"].as_u64().unwrap_or(10) as usize;
+
+        match infer_all_relationships(&self.store) {
+            Ok(mut relationships) => {
+                // Limit results
+                if relationships.len() > limit {
+                    relationships.truncate(limit);
+                }
+
+                json!({
+                    "success": true,
+                    "relationships": relationships.iter().map(|r| {
+                        json!({
+                            "entity_id": r.entity_id,
+                            "interaction_count": r.interaction_count,
+                            "avg_priority": r.avg_priority,
+                            "days_since_last": r.days_since_last,
+                            "bond_strength": r.bond_strength,
+                            "relationship_type": r.relationship_type,
+                            "confidence": r.confidence
+                        })
+                    }).collect::<Vec<_>>()
+                })
+            }
+            Err(e) => json!({
+                "success": false,
+                "error": e.to_string()
+            }),
+        }
+    }
+
     fn handle_unknown_method(&self, id: Value) -> Value {
         json!({
             "jsonrpc": "2.0",