Kill 100 enemies during the credits to get a different ending screen.

Press R1 or L1 at the mission select screen to get different characters:

Beat the game on easy for shirtless Dante

Beat the game on normal for Devil May Cry 1 Dante

Beat the game on hard for shirtless Devil May Cray 1 Dante

Beat the game on hard to unlock the Legendary Dark Knight

Beat the game on hard to unlock Sparda

Tap L2 after firing a slow to reload weapon (i.e., shotgun) and flick it back to fire it again. It is slightly faster than waiting for the weapon to reload.

Die three times in normal mode.

At the start menu hold L1 + L2 + R1 + R2 then rotate the Left Analog-stick. You will here a voice say "Devil May Cry and all modes, costumes, and galleries will be unlocked.

when using Ebony & Ivory and the shotgun, you can shoot the shotgun faster by firing, switching to Ebony & Ivory with L2 (default), firing once or twice with them, then switching back to the shotgun. If timed correctly, you can fire the shotgun as if it were one of your pistols. Note: This trick can be used with other weapons, such as Artemis, but may not work as well.
You can glide while in demon form by having the Rebellion equipped. Hold R1 + X to glide. This move will not appear in the library, but is automatically learned. Alternately, you can purchase the Air Raid maneuver for the Nevan weapon.

Kill all three vergils with the dante without a shirt then when you choose your skin vergils will be there.

Beat Dante must die mode to unlock the Devil Trigger. er 300, 800, 1000 or 1500. She will randomly appear in one of them when you visit the shop.

When loading, you can press either 'triangle' or 'square' button to slash or shoot respectively at the 'NOW LOADING' word. Many other button combinations (using buttons and the analog stick) can be done during the Now Loading screen as well so experiment.

To get easy orbs very early in the game, enter Secret Mission 1 (by entering metal sliding door off to the right before you fight Cerberus). Re enter it as many times as desired. After completing it once, you will gain a huge Red Orb which is worth around 400 orbs.

A great place for orbs is the fifth secret mission found on stage 9. Go through the level untill you get to the area where you get the spiral. Go through the passageway underneath the waterfall and up through a small tunnel directly across from where you. The secret mission is marked by a few bright glowing crystals. Once inside you must destroy every breakable object within 40 seconds. The jukebox contains 500 red orbs, although there are many other things to smash as well. Since you can repeatedly pass/fail this mission and not encounter any enemies, you can repeat this process for lots of easy orbs!

Shotgun - Inside the bar in mission 3. This is after unlocking the blue exit door.
Artemis - Find and use all three essences on the laser machine in mission 5.
Spiral - The outdoor area in mission 9 has a high area near a waterfall; jump to the high area for the gun.
Kalina Ann - Defeat the mission 16 boss.
Gallery - Complete Easy or Normal
Gallery update - Complete Hard
Hard - Complete Normal
Dante Must Die - Complete Hard
Heaven or Hell - Complete Dante Must Die
Quicksilver style - Defeat Greyon at the end of mission 12
Doppleganger style - Defeat Shadow enemy at the end of mission 17