Boss Rush Beat the game with best ending (All bosses)
Julius Mode Finish the game with the bad ending
New Game + Beat the game with best ending (All bosses)
Sound Test Mode Complete the game with the best ending and the Sound Test Mode will appear on the Main Menu.

Rare ring at start  -  Have Aria of Sorrow in the GBA slot when creating a new game. You will start with the rare ring item in your inventory.

Completion bonuses:  Successfully complete the game with the best ending to unlock the Boss Rush mode, Sound Test mode, and New Game+ options.

Successfully complete the game under the hard difficulty setting as Soma to get the Hell Flame Soul move.

Successfully complete the game with any ending to unlock Julius mode.

Alucard in Julius Mode:  To get Alucard as a playable character, enter the Demon Guest House from the lower right enteric. When you enter, Alucard will have a conversation with Julius and he will join your team. Alucard has his Alucard Sword and the High Jump Boots. He can also turn into a bat. He can use his Hellfire ability to attack from afar.

Potion:  Successfully complete Boss Rush mode.

Terror Bear:  Finish Boss Rush mode in less than eight minutes to get the "Terror Bear" weapon.

Nunchakus:  Finish Boss Rush mode in less than seven minutes to get the "Nunchakus" weapon.

Death Robe:  Finish Boss Rush mode in less than six minutes to get the "Death Robe". It a powerful armor, but Dracula's Tunic is better.

Defeating the Iron Golem:  Use the Imp soul. It switches HP damage to MP damage.

Attack faster:  If you have a heavy weapon like a great sword, slow attacks can be a problem. To hit the enemy twice quickly, stand very close to them and attack. Then just as it connects, press L to dash backwards. As soon as it stops, you can attack again; and if you are still close enough it should hit. This is quicker than just attacking twice normally.

Last two abilities:  The last two abilities are actually enemy souls. These souls are the Procel and Mudman. Procel is found in Subterranean Hell. whereas Mudman is found in the Abyss. These souls allow you to walk through water and sand freely.
 
Rocket Launcher (RPG):  Finish Boss Rush mode in less than five minutes to get the Rocket Launcher "RPG".

Chaos Ring:  Get 100% of the Souls to get the Chaos Ring, in the abyss.

Worst ending:  Defeat Dario Bossi during the second battle.

Bad ending:  Use the Paranoia Soul to enter the mirror during the second battle with Dario Bossi. Defeat Aguni, then return to the other side of the mirror and get the Magic Seal. Then, go to the center of the castle and enter the Boss door without Mina's Talisman.

Best ending:  Use the Paranoia Soul to fight Aguni. Collect the Magic Seal then return to the center of the castle. Have Mina's Talisman equipped when you enter the Boss door, then complete the game.

Easier Boss Rush mode:  Activate the Devil Soul and use the Valmanway Sword. You will take down all the Bosses in less than five minutes.

Defeating Dmitri:  If you have a Witch Soul, let Dmitri copy it. The projectile attack from this soul is fairly easy to avoid and will make this fight a lot easier.

Use Une on him and let him copy it. It is the easiest enemy skill to avoid. Also, be sure to not use any R skills (especially the familiars), as he will also copy them and use them along with the bullet skills, giving him two attacks which you must avoid.

Secret Monsters:  There are three secret monsters (the Yeti, Mothman, and the Flying Humanoid.) To get the Yeti you must first get the Waiter Skeleton Soul then go back to the very beginning of the game. You will see him walking in the background. Toss the curry and he will come up where you can attack him and gain his soul. You will need the Rycuda soul leveled up to at least two to get Mothman. Go to The Pinnacle to the area where there are two final guards and a light in between them. Strike the generator next to the light with Rycuda and Mothman will appear. For the Flying Humanoid, you will need the Mandragora soul. Enter the room in between the Zephyr Boss room and a save room then use the soul.

Easy Soul Eater Ring and money:  Get the Mimic soul and go to the Clock Tower. There is a save room with a spike pit next to it at the bottom right corner of the Clock Tower. The Mimic soul will usually give you 500 gold each time you are hit by the spikes. Whenever you get low on health, go to the save room, heal, and repeat.

Double Archer:  Set Amalaric Sniper soul as a Bullet soul, then set Skull Archer as a Guardian soul. Whenever you go to charge the Skull Archer with R, the Amalaric Sniper will automatically activate at the same time and fire alongside. This is a very powerful combo, especially in the early levels, as how most enemies are not strong and will die quickly. Also note that depending on how long you charge the Skull Archer's soul, it may consume a lot of mana.

Soul Synthesis warning:  If you want the best weapons in the game, be ready to give up some of your Boss souls. For example, if you want the Claimh Solais then you must give up the Gergoth soul.

Easy experience:  Equip yourself in the A set with Dracula's Tunic or Demon Mail, Gungner the Last Spear (recommended due to its reach and does not lose important souls) or Valhmanway Sword, and the Shaman Ring. In the Soul part, equip the Imp soul in Bullet, the Black Panther in Guardian, and the Mothman in Enchant. In set B, have all the same except for the Chaos ring, the Succubus bullet soul. Go to the large falling trail near Lost Village (in front of Yoko's shop there is an entrance where you blow up the ceiling in the fountain). Then, go to the end. Go right there and you will see an Iron Golem. With set A, use the Imp Soul and the weapon you selected to kill him faster. When you run low on magic, change to set B with X and replenish fast with the Chaos Ring. If you run low in health, use the Succubus Soul to get life from the ghosts on the right and replenish. Go from right to left (from the ghosts' side) with the Phanter soul to go faster. Kill the Iron Golem to get 3,500 experience points each time.

Free potions:  In one section of Subterranean Hell you will notice a see-saw with a boulder sitting on it. In the background are bells hanging on the walls. Hit the flat part of the see-saw with a hammer type weapon. The rewards for ringing the bells are as follows:

First (lowest): Potion
Second (middle): High Potion
Third (highest): Super Potion