Character.cs

using UnityEngine;
using UnityEngine.AI;

public class Character : MonoBehaviour
{
    private CharacterBrain brain;

    private void Start()
    {
        UseAIBrain();
    }

    private void Update()
    {
        brain.Tick();
    }

    public void UsePlayerBrain()
    {
        brain = new PlayerBrain(GetComponent<NavMeshAgent>());
    }

    public void UseAIBrain()
    {
        brain = new AIBrain(GetComponent<NavMeshAgent>());
    }
}