Larry Jr. is a light tan-colored, worm-like bug that comes in different number of segments. He sports with a blood-stained mouth filled with long teeth. His beady black eyes are also appears to be shedding tears.
Larry Jr. is always paired with duplicates (though sometimes it might appear as one single long chain), and will quickly crawl around the room, leaving behind Poop piles. Larry Jr. has an erratic movement pattern as he appears to ignore Isaac until the player walks directly into his path, similar to the Charger Maggot - which is also similar to Larry in appearance. It also randomly leaves poops on its path which might block the player's way.
Larry Jr. has multiple segments, each that have to be destroyed to kill the boss. Destroying the head will cause the previous segment to become a head, and so on until all of the segments are destroyed. A segment that is not connected to any other segments (due to the middle segment being destroyed) will die instantly, which makes the fight easier for characters with low damage.
Upgrades such as Cupid's Arrow or Technology that add piercing to Isaac's tears make the fight much easier, as all of the segments take damage.
This also applies to all items with explosive potential, such as IPECAC, which will almost always one-shot Larry Jr. if timed well.
Poison is also very effective, as each segment of his body is poisoned individually. Poisoning multiple segments essentially stacks the poison effect, dealing a lot of damage relatively quickly.In later Chapters, Larry Jr. can appear in normal rooms a mini-boss, usually with less segments.
When fighting Larry Jr. while having the Bloody Lust item, destroying each Larry's piece counts as a kill for Bloody Lust.
When green, he will fire slow-moving and short-ranged shots randomly in whatever direction he's facing at the time. He will also have slightly lower health.
He will be slightly larger, slower, have more health, and not produce any poop piles. A Soul Heart is always dropped after fighting this boss.
Eternal Larry Jr.s will slowly grow new segments. Instead of dying when only a head is left, the head will move at half its normal speed while leaving red Creep where it moves. Single heads are quicker to gain new segments, and will behave normally after one is regained.
Larry Jr. sometimes shows up in other rooms later in the game starting from The Caves up to The Womb. He can appear in groups of three or more, and in its green or blue champion variants. The more Larry Jr.'s appear in the same room, the shorter each one of them is.
From one of the sketches of the artbook, Larry Jr. suppose to pause for a moment with his expression changed while leaving a poop behind in a similar manner of Chub.
Humorously, from the bottom of the page, Larry Jr. can be seen frowning and then smiled.
Larry Jr. appears in the demo of the original game.
Edmund McMillen also confirms it from the artbook at page 1.
In the credits of Super Meat Boy, three "Larries" come out of a dead "Larry". They are believed to be "Larry Juniors".
Larry Jr. is actually crying, unlike most of the other bosses. This is possibly because Larries' Lament was deceased.
When asked about why he was crying, the game's creator Edmund jokingly replied "Because Isaac is crying."
In his boss art, Larry Jr. is not covered with blood, unlike in-game.
Larry. Jr may be a reference to the arcade game Centipede. Larry Jr creates obstacles that makes it harder to shoot him, but instead of barriers, he creates poop.
If Isaac die and respawn in the last room before Larry Jr. with 1UP or Ankh, the poop will remain but can't be touched.
Very rarely, the segments of Larry Jr.'s body will glitch causing all the the body segments aside from the head to stack on a single spot, making his body appear to only be a head and a single segment attached to it. It is possible to observe this effect as the back end of his body will not flash red when hit, which is a result of the body section you are actually hitting being behind the foremost section. This can always be observed in the Boss Challenge Room.
If Larry Jr. creates a pile of poop on another Larry Jr. as the other Larry Jr. is turning, the other Larry Jr. gets frozen creating an infinite poop generator. If grinded long enough, one can obtain an infinite amount of health and money from this regenerating pile of poop.
If Wrath of the Lamb installed, the Green Champion version of Larry Jr. will get the head sprite of The Hollow if he fires a shot upwards.
Interestingly, the green champion version of The Hollow uses Larry Jr.'s head sprite when firing instead of its own.
Rarely, for an unknown reason, one of the Larry Jr. can pass over a rock if there is one in the room.
If, when Isaac enter a room with Larry Jr., if there is a 5 segments one and a 3 segments one, if shot the middle segment of the 5 segments Larry Jr., it will split in another 2 Larry Jr.'s, so if Isaac didn't killed the 3 segments one, Isaac will have more trouble; there will be 3 Larry Jr.'s.