We first learn about the Half-Blood Prince through indirect characterization via the potions book that Harry uses. We learn about all of his wit, cleverness, and undeniable talent for potion making without actually meeting the Prince. I think this was a very clever way of JKR to allow us to learn more about Snape and to see him in a different light. He was almost a friend to Harry thoughout the book, and was there for him at every turn, without actually being there.
While Snape seemed almost friendly as the HBP, what I don't understand is
how he became so powerful and so angry at such a young age. It is my belief that it has to do with his parenting. Many of the most powerful wizards and witches have one magical parent and one muggle parent. Voldemort, Harry, and possibly Dumbledore are all good examples of this. Snape's parents fit the bill and his extraordinary talents fit as well. Perhaps this magical/muggle mixing has a special sort of effect on the child.
I believe that Snape's talents were increased and honed due to the fact that he was tormented in school. I think that his unpopularity caused him to experiment with different potions and spells. How else could he have come up with Sectumsempra and the several corrections and adaptions to the potions??
I think it's intriguing that Snape had such great powers at such a young age. It seems to almost parallel Voldemort to me. Perhaps a future theory...
