CharacterControlSwapper.cs

using UnityEngine;

public class CharacterControlSwapper : MonoBehaviour
{
    private Character[] characters;

    int characterIndex;

    private void Awake()
    {
        characters = FindObjectsOfType<Character>();
    }

    private void Update()
    {
        if (Input.GetKeyDown(KeyCode.Space))
        {
            SelectNextCharacter();
        }
    }

    private void SelectNextCharacter()
    {
        characters[characterIndex].UseAIBrain();

        characterIndex++;
        if (characterIndex >= characters.Length)
            characterIndex = 0;

        characters[characterIndex].UsePlayerBrain();
    }
}