@Tanja1986 I would suggest reading through this reddit thread: https://www.reddit.com/r/Sims4/comments/vewid9/comment/icsqmya/?utm_source=share&utm_medium=web2x&context=3
Essentially if you add him into your household with cheats, another Greg will NOT spawn. I guess there's only one him! But what makes Greg special is that he has a special trait called "Greg trait". This trait is what makes sure he resets and acts all angry and moody.
You can also remove the trait and he becomes more like a regular sim but he still has anger issues (lol). I don't know how this could affect the game but I will give it a shot later sometime and see how it turns out. I have a story in my head where eventually he wants to have a family and after some work might decide to join the household. Based on the lore it seems he can't control himself :( Aveline pretty much confirms that he just looses himself in anger.