This can be helpful in better protecting your health from being less than full to attack the stronger enemies at the latest stages of the game.
It also enables you to get more health from health globes dropped by enemies you kill.
In addition to this, it also helps you to have a high amount of HP without needing to equip or use other items which can help you increase your damage output and survivability.
When you enter a new floor, it drains an item from the room and gives you its bonuses for 15 seconds. As with any Mimic Friend item, if you are poisoned or cursed when your health reaches zero, the ring will leave you.
This ring will turn the Bullet that hit the player’s chest into health drops. With this ring, our heart will become a machine gun!
This is because the chest machine gun will deal damage to all enemies around it, so it instantly becomes a very powerful weapon.
Chest mimics are enemies in the game, but you can instantly kill them with this item by making one chest mimic another chest.
This debuff grants the player it with a 20% chance to drop a heart upon shooting an enemy. This effect stacks up to 4 times if the boss has been killed multiple times.
However, it can only stack from killing unique bosses from specific worlds. The debuff will last until the end of the level.
Activating the item causes Isaac to drop a blue ring that, when picked up, will cause one random pickup on the floor to be replaced with a chest. These chests will have an animated chest icon above them just like regular chests do.
Usually, the effect lasts for 15 seconds, but if Isaac were to pick up any of these chests before the effect expires, it will refresh to its full duration.
Hearts will double only if the player is dealt damage at the same time that another player would have been dealt damage by an enemy projectile. Heals will not trigger the ring.
I was a bit disappointed when I came across a Mimic for the first time. Initially I thought it would change appearance or be something more but later I found out that it's just a chest with a healing effect. So is the Ring of Mimic Friendship really needed ?
Turns out, there is a big difference between mimics in vanilla mode and Hardmode.
In Hardmode they break cursed chests which emit poison gas clouds that damage you when you go near them.
It's best not to open them in general though but in case you need to farm cursed hearts from…
Just in case someone else wants to know how this works, I think the item's effect only applies if you are already at full health. For example, I got hit for 1 damage in the video below, then the ring took effect and healed me for 1/2 a heart.
I'd be interested to see if it were possible to get the ring effect after taking an extremely small amount of damage (e.g. shot from a bullet that inflicts 1 damage).