MapleStory/Familiars

Crusader Codex is essentially a book which every player is given that catalogs monsters. When the Crusader Codex is added to the game, monsters begin dropping "cards" that contain detailed information on them. Each individual monster's card can be collected only once, within the card has the following details:
 * 1) The monster's slot in the book lights up and it's standing sprite becomes viewable.
 * 2) The monster's HP and MP values will be displayed, as well as some new animations of the monster, such as walking or knockback.
 * 3) The monster's level will be added.
 * 4) A complete drop list is revealed, with certain items (such as event items) omitted.

Though cards will continue to drop after the card is found, picking up more will not do anything other than give you a message saying you can't pick up more than one. In addition to their permanent change to a player's monster book.

Cards in the book are sorted by relative level range. There are tabs on the side of the book which allow one to skip from level range to level range. The tabs on the book's side are color coded, and the colors match those in the background of the cards which appear in that section. A Chief Gray, for example, has a green background on its' card, and the tab under which it appears is also green. The only exception is cards from Boss monsters, as these appear in a different tab, called "Skull Tab" and has an image of a skull. Certain event monsters also appear as special cards.

Monsters found in Maple Island, Victoria Island, Ossyria, Masteria, Zipangu and Party Quests drop cards. Any new monsters released shortly after the release of Crusader Codex do not have cards. In certain occasions, you might have already killed a monster before the launch of Crusader Codex and the ability to enter the place to get the card is blocked because of your level range, you may get it through an NPC (for example, in Mushroom Kingdom, the Mushking allows you to get the card if you had already completed the quest before hand).

Card Sets
Sets allow you to get stat bonuses by obtaining cards that the set requires. For example, the set Victoria Island Friends requires you to get Snail, Blue Snail, Red Snail and Shroom to be able to get the set. After getting the cards, you can select the set to be activated in which you will be able to get the bonus as stated.

Familiars
Monsters are now able to drop familiar cards. These are Use items that enable players to summon the monsters to fight alongside them. Three cards can be found per monster that drops familiar cards; each use of the card increases the monsters' total number of vitality orbs by one (for a total of three at the third level). Additional cards can be picked up, but cannot be used. Also, cards past the second do not affect the level, strength, range, or special abilities of the summon.

Unlike a pet, familiars cannot be enhanced by items or fed. They also have vitality orbs, which indicate their health. Every time a familiar attacks, its vitality drops. Some monsters' vitality drops as a result of their special ability.