Question Newbie question but how do I make an enemy target a singular person?

I'm trying to make an enemy that targets one person until someone else attacks it, the one person would be the person who activated the enemy by getting close to it, how would I do this?

Code (CSharp):

1. using System.Collections;
2. using System.Collections.Generic;
3. using UnityEngine;
4. using Photon.Pun;
5.  
6. public class DefaultEnemy : MonoBehaviourPunCallbacks
7. {
8.     public float speed;
9.     public float activateDistance;
10.      
11.     private bool activated;
12.     private GameObject[] networkHeads;
13.  
14.     private void Update()
15.     {
16.         networkHeads = GameObject.FindGameObjectsWithTag("Network Head");
17.        
18.         foreach ( var head in networkHeads )
19.         {
20.             if ( Vector3.Distance(transform.position, head.transform.position) > activateDistance )
21.             {
22.                 photonView.RPC("EnableEnemy", RpcTarget.AllBuffered);
23.             }
24.         }
25.  
26.         if ( activated )
27.         {
28.             // do stuff
29.         }
30.     }
31.  
32.     [PunRPC]
33.     void EnableEnemy()
34.     {
35.         activated = true;
36.     }
37. }