Ender's Game is fairly weak on the aliens front. They make no direct appearances until the very end of the book.
The (unrelated) aliens in the sequel Speaker for the Dead are considerably more interesting. I wouldn't recommend any of the other sequels/prequels/sidequels, though.
The (unrelated) aliens in the sequel Speaker for the Dead are considerably more interesting. I wouldn't recommend any of the other sequels/prequels/sidequels, though.