Dostoevsky says in his preface that Alyosha is his novel's 'hero'; he's obviously meant to be (at first sight) the 'nice guy' out of the three brothers.
But do you think it's the case that he was created for this purpose, as a 'traditional' hero, the good guy whom we should like and be behind? Personally, I think - given Alyosha's religious convictions, and the fact that Dostoevsky was not a religious man but would have liked to be able to have faith - that he's the sort of guy FD wishes he could be.
Perhaps also FD based Alyosha on the person he wanted his son* to grow up to be. Certainly there must be some significance in the fact that he named him after him.
But those are just some thoughts of mine
* I'm sure most people know this, but Dostoevsky's son, Alexei died at a young age just as FD was starting to write BK.