When he was a child, Harry encountered an injured snake. Moved by compassion, he tried to help it as much as he could. The snake was in fact a mountain veela from Bulgaria, who had escaped from trouble and ended up wounded and exhausted in Britain. She had transformed into a snake to stay hidden.
In snake form, the veela befriends Harry, and eventually reveals herself to the child. Mountain veela typically have the gift of prophecy, and this one is no exception. She recognizes the strands of fate around the child, and decides to stick around to help him.
It is quite a different Harry Potter that meets Fleur Delacour, and the mountain veela is delighted when she hears that the boy whom she now sees as an adoptive son has a crush on another veela.