Multiplayer in The Binding of Isaac explained, includes a baby generator.
One of the biggest selling points for the upcoming remake of The Binding of Isaac is its new multiplayer mode wherein you and one other person can face the dangers of the cellar together. Thanks to a helpful explanation on the game’s website, we now also know exactly how this new feature will work.
A second player can press start on their controller (both on consoles and on PC) to spawn as a small, floating baby that resembles a familiar. To do this, the second player consumes one of the hearts of the first player and borrows it until they are killed or decide to leave the action for a moment. This baby can fire projectiles and has all the items (including usable ones) that the first player had upon its creation, though stats for both players will be balanced to preserve the challenge.
Since randomization is a big part of Isaac, the type of baby that the second player will spawn as is randomized as well. Its appearance can be completely cosmetic, but some “special babies” will have specific powers. If you want to read more about the specifics, then you can find all the information and some gifs in the source below.