re: Assassin's Creed 2

From what i've grasped so far Da Vinci seems to have been a friend of the family and he helps you throughout the game. His inventions add multiple aspects to the gameplay. So far he's
1.fixed the hidden blade
2.added a shield to the blade
3.allowed yo to have two blades(assassinate two people at once)
4.different assassination tech.
He does this by deciphering the codex pages you bring to him.