During the second battle with Dario Bossi, simply beat Dario.
During the second battle with Dario Bossi, use the Paranoia soul to enter the mirror. After you beat the new boss, Aguni, go back to the other side of the mirror and get the magic seal. Go to the center of the castle and enter the boss door without Mina's tailsman equipped.
Use the Paranoia soul to fight Aguni, pick up the magic seal, and go back to the center of the castle. When you enter the boss door have Mina's talisman equipped. You will now get a more extended version of the game and all you have to do is beat it.
Beat the game with the good ending (beat Menace), after that, you can choose to start a new game + on that file in wither Normal or Hard difficulties.
Hell Fire Soul
Complete Hard Mode with Soma and you will get the Hell Fire Soul (Dracula's Triple Fireball).
Play as Julius Belmont
Finish the game with the bad ending.
Play as Alucard
Alucard joins Julius Belmont and Yoko to hunt down Soma Cruz in the Julius Belmont mode. Find him in the Demon Guest House, in the same room where he gave Soma Mina's Talisman during. You can switch between Julius, Yoka or Alucard.
Sound Test Mode
Complete the game with the best ending and the Sound Test will appear on the Main Menu.
Boss Rush Mode
Beat the game with best ending (All bosses).
Collect all souls to unlock the chaos ring. It will appear in one of the blue rooms of the abyss. It gives the user extreme MP regeneration (practically infinite) and a stat boost.
In the left part of subterranian hell, there is a slot machine door, make sure your last 3 digits of your gold are 666, the door will slide back, and you can get the gold ring, making monsters/candle drop more money.
Rare ring at beginning of game
This only works if your DS has a Game Boy Advance slot. Have Aria of Sorrow in the GBA slot when creating a new game. You will start with the rare ring item in your inventory.
Fight the Yeti
Found at the very beginning of the castle, in the snowy field just outside the castle walls. You can see the Yeti walking around in the background, but to lure it out you'll need to throw out some curry using the Waiter Skeleton soul.
Fight the Flying Humanoid
The Flying Humanoid is located at the very top of the Cursed Clock Tower, just outside the save room. You can see the enemy flying around a spire in the distance. To kill the enemy, use the Mandragora soul. You'll instantly earn this enemy's soul.
Fight the Mothman
Before you can lure out the Mothman, you need to first get the Rycuda soul. With that power, head to The Pinnacle and find the long hall that's guarded by two Final Guard enemies. Between them is a generator you can hit with the Rycuda soul attack to turn on a light, luring out the Mothman.