An Esper Monster is a type of Monster Card. The color of their frame is silver, similar to a Token Monster color scheme, but with a metallic pattern and borders that fade to black. These cards are placed in the Extra Deck.
Like any other Extra Deck monster, Esper Monsters cannot exist in the hand or Main Deck under any circumstances, and must be returned to the Extra Deck if they were to be moved to the hand or Main Deck.
A lack of Levels
Esper Monsters do not possess Levels or Ranks or Link Ratings. Because Esper Monsters do not possess Levels, Ranks or Link Ratings, they cannot be used as material for a Ritual, Synchro, Xyz, or Link Summon in certain instances. Furthermore, they are unaffected by cards that modify or affect Levels/Ranks/Link Ratings.
TYPE-A and TYPE-B
All Esper Monsters are classified as TYPE-A or TYPE-B in place of where a Monster Card's Levels are normally located. TYPE-A is denoted with a red circle with a white "A" in the center. TYPE-B is denoted with a blue circle with a white "B" in the center.
TYPE-A Esper Monsters can be Esper Summoned during your Battle Phase in an Open Game State.
TYPE-B Esper Monsters can be Esper Summoned during your opponent's turn in an Open Game State, whenever priority passes over to you. This will usually be accomplished prior to both players agreeing to enter the next Phase in a turn.
Main article: Esper Summon
An Esper Summon is the act of Summoning an Esper Monster from the Extra Deck by using exactly 1 monster listed on the Esper Monster as Esper Material. During the Main Phase, the turn player can Esper Summon an Esper Monster from the Extra Deck by using an appropriate face-up monster. When an Esper Monster is Esper Summoned, the Esper Material is stacked and placed underneath the Esper Monster, similar to an Xyz Monster.
There can only be 1 and only 1 Esper Material attached to an Esper Monster. Any materials attached to a monster used as Esper Material for an Esper Summon are sent to the Graveyard instead. Token Monsters cannot be used as Material for an Esper Summon.
An Esper Material is not treated as being on the field, and any monster used as Material for an Esper Summon is not considered to be leaving the field.
Leaving the Field
At the very end of a turn, before the next player begins their turn, all face-up Esper Monsters in a Monster Zone must be returned to the Extra Deck, and any Esper Material attached to each monster is placed in their controller's Main Monster Zone(s). Esper Monsters cannot be Special Summoned from the Extra Deck via their own Summoning method when this occurs.
If an Esper Monster was used as Esper Material for for an Esper Summon, that Esper Monster must also be returned to the Extra Deck at the end of the turn, even if it has no Esper Material.
Monsters returning to the field this way are not treated as being Summoned nor returning to the field because of a card effect.