A large stitched doll made in Isaac’s likeness.
Upon activation, Isaac will drop a dummy replica of himself that functions like a Bomb, which enemies will focus on instead of Isaac. After 5 seconds, the replica will explode in a large blast radius.
Any orbital Familiars with Isaac, will be near Best Friend until a bit after it explodes.
- Since Best Friend is treated as a large bomb, it can be pushed around by Isaac's tears or movement in addition to enemy tears or movement. Having any orbital Familiars other than Distant Admiration and Forever Alone prevent Isaac's ability to move it (not affecting enemies or Bobby-Bomb). Guppy's Hairball can move Best Friend with momentum as long as you don't have orbitals other than Distant Admiration and Forever Alone.
- In terms of damage and properties, Best Friend's explosion is the same as the large bombs from Mr. Boom/Mr. Mega.
- Swallowed Best Friend won't get a chance to deal damage with orbitals, but you immediately regain the ability to drop Best Friend (if you have the charge for it) and orbitals return to the player.
- Isaac gains a short period of invincibility frames while activating Best Friend, which can be used to prevent damage when activated at the right time.
- The decoy feature of Best Friend does not work on Hellfireplaces and Super Troll Bombs.
- The Dople/Evil Twin will follow Best Friend's location when it is dropped, warping to where it should be when it explodes.
- Guppy's Hairball doesn't warp back so much as rapidly travel to the player (hitting things along the way) after Best Friend is used.
- The Level 2 Cube Of Meat will fire normally while orbiting Best Friend.
- Best Friend's 'instant kill' cannot apply if you have no red heart containers. This gives ??? a natural advantage with it.
- Bob's Curse - Poison effect also applies to Best Friend.
- Bobby-Bomb - Homing effect also applies to Best Friend.
- The Virus - The invincibility frames for activating the item allow a free hit from The Virus.
- Orbitals in general synergize well, though Pretty Fly and Halo Of Flies might not do much for it beyond blocking some tears (and harming fly type foes) that might have been heading your way.
- Best Friend has a glitch where certain hits will harm you unconventionally while it is active. Some will cause red heart damage and others soul/eternal heart damage. For Red Heart cases, the player must be hit by the attack (hitting just Best Friend will do nothing). The damage value of the attack deals itself in red hearts to the player (which is fatal in most cases due to far exceeding red heart limits). Red heart reduction to zero will kill you. For Soul/Eternal Heart cases, Best Friend must be hit (hitting only the player won't cause a soul hit). Whatever damage in hearts that source would be worth is taken from you (affected by The Wafer) while Best Friend is destroyed. No amount of these can kill you and invincibility frames can negate the damage.
- Red Heart cases: Eye/Bloodshot Eye (laser), Mom/The High Priestess (stomp), Krampus (laser), The Fallen (laser), Satan (laser and death explosion), Monstro II (laser), Blighted Ovum (laser), The Bloat (laser), Daddy Long Legs/Triachnid (stomp), Pride/Super Pride (beams), Gluttony/Super Gluttony (laser), Isaac (boss)/??? (boss) (beam), Conquest (beam), Bob's Curse (poison), Bob's Rotten Head (poison) and IPECAC (poison).
- Soul/Eternal Heart cases: Eye/Bloodshot Eye (laser), Mom/The High Priestess (stomp) and Chubber (shot).
- A Vis/Double Vis' laser cannot harm you while Best Friend is out.
- Satan's stomp can harm you while Best Friend is out (no bugged hit mechanics though). This also happens with Mom's stomp and The High Priestess.
- Poison sources deal seven red hearts of damage per tick (number of times depends on the source hit with). Leaving the room early can avoid the poison damage and end the effect. Letting the duration run out before placing Best Friend (where applicable) can also avoid the damage.
- Isaac/???/Conquest beams deal two red hearts per hit from a beam.
- Swallowed poison bombs don't afflict poison, so they cannot bug hit Isaac for several hearts of damage per tick.
- Mom won't take damage from Best Friend's explosion if triggered by the stomp.
- Soul/Eternal Heart bugged hit sources can be exploited when you have no soul/eternal hearts (running on nothing or just red hearts) to activate player damage based items/trinkets like Fish Head, Red Patch, Polaroid, Celtic Cross, Dead Bird, Infestation, Missing Page, Holy Water, etc. (not exactly a tactic you want to rely on though).
- A shooting Familiar will become Brother Bobby when the item is active, then revert shortly after it explodes. If you already have Brother Bobby, the it switches places with whichever familiar it would otherwise have replaced. The priority for replacement is (highest to lowest): Isaac's Head, ???'s Soul, Little Steve, Demon Baby, Little Gish, Rainbow Baby, Harlequin Baby, Ghost Baby, Sister Maggy, Robo-Baby and Abel. Little Chubby prevents this effect while passive (though an immediate pick-up won't get this effect until the next room).
- The Cursed Skull has a glitch where if a soul/eternal heart bug hit leaves you with a half soul heart or more while holding Cursed Skull, you become invincible for the remainder of the room. The only means to die would be a red heart bug hit. If lacking an appropriate foe to set this up, The High Priestess can often fill the role.
- Best Friend pulls Bum Friend when no enemies are around. Exploding on it seems to remove it for a room.
- Daddy Long legs will attack the player while Best Friend is active (no damage/hit).
- Visual bug with red explosion on Best Friend in some case (lack specifics).
- Visual Bug where you'll see the type of bomb Best Friend is imitating during a bugged soul/eternal heart hit case.
- Visual bug with blue limbs on Best Friend (seemingly in alt. rooms only).
- If you summon Monstro with Monstro's Tooth, he will still aim at you.