Whenever Isaac takes damage, he summons a micro-Isaac familiar, known as Minisaac, that chases nearby enemies and attacks on its own.
Minisaacs deal 1.35 base damage that scales with +0.15 per floor: damage = 1.35 + 0.15 * (floor - 1)
Minisaacs shoot 3 tears per second and their tear delay is not affected by Isaac's stats.
Minisaacs persists between rooms and floors.
Each Minisaac is spawned with 15 health points that are constantly drained with a rate of 0.4 per second. The draining stops when they reach 0.8 HP.
They are healed back to full HP whenever Isaac takes damage, even when the familiars cap is reached.
Enemies can damage Minisaacs by shooting projectiles at them or dealing contact damage.
Isaac's bomb explosions will also damage the familiars.
Minisaacs are immune to spikes and lasers.
Notes
The familiar cap for Giant Cell is 16, after which taking damage will not spawn a new Minisaac.
Minisaacs will change their skin color based on the character that they were spawned from.
Certain items that change skin color can also affect the familiars' appearance.
When playing as The Forgotten, the familiars take the style as either The Soul or The Forgotten, depending on who gets hit.
If the familiar takes the style of The Forgotten, it will also inherit its bone club attack, dealing 8.1 base damage that scales with +0.9 per floor. It will swing its bone club 1.5 times per second. It can also interact with pickups using the bone club.
Minisaacs for Tainted Forgotten will spawn from The Forgotten's body and inherit its skin color but they will shoot regular tears.
If Isaac has any form of flight, the Minisaacs will receive wings and will be able to fly as well.
Minisaacs spawned before flight was acquired will not get upgraded, unless the flight is acquired by killing Dogma and starting The Beast fight in which case they will get upgraded.
Minisaacs that acquire temporary flight (e.g. from The Bible) will remain permanently upgraded until they die, even after the effect of The Bible expires.
If being hit makes Isaac become the Whore of Babylon, a Shadow-like Minisaac will spawn instead of a regular one.
Synergies
BFFS!: Increases damage dealt by the familiars by 40%.
Dull Razor: Provides a penalty-free way of spawning Minisaacs and/or healing existing ones.
Glowing Hourglass: Each use of Glowing Hourglass that does not move Isaac to a different room than the one he is currently in (such as the starting room of a new floor) will duplicate the Minisaacs. (This is possibly a bug)
The Forgotten, the familiars take the style as either The Soul or The Forgotten, depending on who gets hit.
Tainted Forgotten will spawn from The Forgotten's body and inherit its skin color but they will shoot regular tears.
Azazel or 
Tainted Azazel will not inherit the 
Brimstone laser and will fire regular tears.
Dogma and starting 
The Beast fight in which case they will get upgraded.
The Bible) will remain permanently upgraded until they die, even after the effect of The Bible expires.
Whore of Babylon, a Shadow-like Minisaac will spawn instead of a regular one.
BFFS!: Increases damage dealt by the familiars by 40%.
Dull Razor: Provides a penalty-free way of spawning Minisaacs and/or healing existing ones.
Glowing Hourglass: Each use of Glowing Hourglass that does not move Isaac to a different room than the one he is currently in (such as the starting room of a new floor) will duplicate the Minisaacs. (This is possibly a bug)
Sacrificial Altar: Converts all Minisaacs into 1 
Penny each upon use.
The Common Cold / 
Ipecac: Minisaacs will be colored green but they will not get any benefits nor shoot out poison tears.
Dummies on screen will not only be attacked by the Minisaac spawned by Giant Cell, but will prioritize attacking the Dummy over any other enemies.
The Binding of Isaac: Rebirth
