The question seems to assume that there is only one reason for why God allows evil. Why think this is the case? It isn’t prima facie obvious. The atheist needs to give us reason as to why there would only be one reason.
One reason is to build character. The atheist will object that this doesn’t account for every case. But so what? We could apply another reason for the cases that character building doesn’t cover. Like free will.
Another reason God allows evil is for justice. Now I’m not saying this is the norm. And I am not claiming that we should say this or even think this when a tornado strikes and decimates a town.
Furthermore, God allows evil for his redemptive work in human history. On Christianity, evil plays a huge role on the Cross of Jesus Christ. There Christ declared victory over evil and suffering. God became a man in Jesus and suffered like us and for us. If God did indeed do this, then how can we rationally doubt his reasons for allowing evil? How could we not trust him?
A fourth reason for allowing evil is that God a greater good/goods ultimately come from suffering and evil that wouldn’t ultimately be logically possible if evil and suffering and evil didn’t exist.