Why should the Spiritual Friend be a human being, why not a pet, for example?
You will develop a stronger feeling of Metta with a personal or human connection. That is also why we want the person to be alive and not dead.
We want them to be a friend of yours. You could also choose a famous person as a Spiritual Friend, but again, it’s not as good as picking someone you know.
We want them to be same sex so no lust arises. Or at least someone where you know lust would not arise – neither lust in you, nor lust in the Spiritual Friend you chose.