Successfully complete the game in hard mode and get at least a "Regular" rank overall to get the best ending. This includes an extra conversation between Vanessa and "The Client", with Vanessa apparently in the shower. You also will get a new "Game Over" screen.
Better ending
Successfully complete the game under the normal difficulty setting to get an extra conversation between Vanessa and "The Client"; a new end screen saying "Game over...", and a new title menu.
Blackbird Suit
Successfully complete the game under the easy or normal difficulty setting to unlock the Blackbird suit. It is maxed out on every level and has three super moves from the start. You cannot change those moves or upgrade the suit. You also get a new title menu when you unlock this suit.
Easy money
Play the trial missions as many times as desired to get extra money to upgrade your suits.
Hard mode
Successfully complete the game under the easy or normal difficulty setting to unlock hard mode.
New Game+
Successfully complete the game under any difficulty setting to unlock the New Game+ option. This allows you to play the game again starting with 0 points, but with all suits as they were in your previous game.
Papillon Suit
Successfully complete the game a second time on the same saved game file. This suit is also maxed out but has no barrier; one hit and you are dead. However, you will get to see a lot more of Vanessa. You can also now assign what super moves you want your character to have at each portal and in the inter-mission menus. When you unlock this suit you also unlock a new title menu.
Thunderbird Suit
Successfully complete the game with all suits in your inventory under any difficulty setting. Your will get your P.N.02 suit called the Thunderbird. It is all black. Get it maxed out in upgrades to get a new title screen.
Viewtiful Joe trailer
Go to the options screen then to the gallery to see a Viewtiful Joe trailer.
Best ending...