Partner System

The Partner Systemis a gameplay mechanic first introduced in Castlevania: Portrait of Ruin, and was later implemented in Castlevania: Dawn of Sorrow, and otherwise serving as an inspiration for the multiplayer Castlevania: Harmony of Despair. This system allows the game to have more than one characters being playable at the same time.

Dual Crush
Dual Crush is a special ability that allows the characters to combine their attacks in order to deal massive damage on a group of enemies. This was present in Portrait of Ruin and Harmony of Despair.

Castlevania: Portrait of Ruin
The Partner System is the bread and butter of this game's combat. The player can call or cancel an ally using the A button. It is also possible switch to a character any time using the Y button. Many relics in this game utilize the Partner System as they provide commands suitable for dual character control, such as jumping over another's shoulders, using an attack, or simultaneously pushing a large object. The HP and MP bars are shared as one, as the player controls one character, the other becomes AI-controlled. Only the player controlled character gets damaged, and the AI-controlled character does not. Suppose the AI-controlled character has run out of HP, he/she will vanish and the player can simply call him/her again. Game Over only happens if the player-controlled character runs out of HP. The two other modes likewise implement this;
 * Sisters Mode
 * Richter Mode (Richter Belmont and Maria Renard)

Castlevania: Dawn of Sorrow
In Julius Mode, Julius Belmont is able to recruit both Yoko Belnades and Genya Arikado/Alucard in his party. The Heads-Up Display from Portrait of Ruin is used as well.

Castlevania: Harmony of Despair
Many characters are present in the gameplay at once since this is a multiplayer game. The Dual Crushes mostly from Portrait of Ruin are implemented here.