Okay, who or what? Well, in Buddhism there isn't a theistic God that controls everything and makes the decisions. Buddhism talks about the Law of Kamma. If you do good, good will come; if you do harm, harm will come. If you're a Lovingkind person then you are going to be reborn beautiful; if you are an angry person then you're going to be reborn ugly. If you are a generous person, you will be reborn rich; if you are a stingy person, you will be reborn poor. If you ask a lot of good questions of your teachers, you will be reborn wise; if you don't ask questions of your teachers, you will be reborn stupid and so on.
According to the scriptures, this is the teaching from the Buddha's own words, so that's fine, but "How come?" is the question. Who or what makes this decision - you can't get that answer. Sorry. The Buddha didn't know the answer. He used to get asked a lot of questions about the big stuff. The beginning of time, how did this whole thing start? I mean, if you really look at this whole thing about being a human being on this planet, it's... it's just... how did this happen? Who made it? Who built this Earth? Who designed our bodies? Was it an architect in a laboratory? You know, who designed this thing? When you really go looking and looking and looking, you come up with a thousand questions and the Buddha himself, said he didn't even know and there's no point in asking the question because you can't get the answer, you can't know who decides Kammic deeds, who made this all up. It's a law. It works. We can't see it lifetime to lifetime because, very few, if any human being knows their past lives.
We can see this law during our lifetime. If you're being generous to other people, do you feel rich? Those moments when you're being stingy and you don't want to give any, do you feel really poor? When you are around somebody who is loving and kind, are they beautiful, despite what they might look like on the outside, they might be full of pimples, a hundred pounds overweight, whatever, it won't matter. If they are loving and kind, you're going to feel they're a beautiful person; it's that simple, the inside stuff shines through.
The scriptures say that from lifetime to lifetime, if you're loving then you will get reborn beautiful, and this law also works in this lifetime as well. Sit down with the best looking person on the planet, Mr. Universe, Miss Playboy, whatever, beautiful specimens from the outside, and watch them get angry sometime, watch them get jealous, watch them when they screw up. Do they become ugly? Yes. So, when we observe this moment-to-moment rebirth, we can see that these mental states create our next "birth". We can see this works on a moment-to-moment level. We can also have a belief or faith that it will work on a lifetime to lifetime level; it stays on that level because we can't know for sure. But as to who or what created this whole thing, we can't get that answer.