How does combining actually work? You can see what the combination is going to result in? And is it a guarantee that its going to be a sss character or is it random?
You combine two cards of a same tier to get a random card of tier above. Combine two S cards to get a random SS; for instance. If you combine two SS cards then it's a guaranteed SSS card. Which particular SSS card is, once again, random.
To combine two cards they need to be in max state and you get them to that state by feeding same or lower level cards.