Gemini are a set of conjoined twins attached by an umbilical cord. Contusion (the large one) has two teeth bucked out and is missing an eye with multiple markings. Suture's (the smaller one) eyes are always closed until Contusion's killed. Their HP is divided equally to each one.
The larger of the two will walk towards Isaac, trying to deal contact damage. Eventually, he starts charging with increased speed for few seconds. Right after charging, he runs out of breath and has to stop, leaving him vulnerable to attack. The smaller one drifts along at a distance, and will fire fast-moving blood shots at Isaac if he comes near and isn't behind cover. The umbilical cord acts like an elastic tether, keeping them in roughly the same area while not restricting range of movement too much.
Killing the larger twin will cause the smaller one to turn incredibly red when becoming enraged, abandoning ranged attacks in favor of charging at Isaac (if the player deliberately focus-fire the small one from the start, it will be killed and they only have to focus on the big one, whose behavior will not change). While enraged, the smaller twin takes a lot more knockback, which can be troublesome if care is not taken.
The character's best option for handling these two is to run around the edges of the room, firing behind them as they go. Gemini's speed is usually not sufficient to reach Isaac, making him easy to fight.
Movement-controlling items like Mom's Bra and Mom's Pad can be helpful if the character find themselves unable to stay ahead of the larger twin's frequent charges. Strong explosive attacks like Bombs are easy to land when the large twin stops for breath.
Due to the knockback from tears, it is possible to keep Suture completely stationary if you got him to run at you parallel to a wall and your tears were being fired straight at him, without influence from movement. (Suture------> <-------You)
Contusion will not react/enraged if Suture was killed first.
Gemini starts off with the two detached; however, the smaller one will act as normal. If enraged by the death of the larger one, it does not move as fast as the normal one. However, there is a chance where just the smaller half of him is green. In this case, it's easier on the player, as when it starts its charge, it'll move slower towards the player and is much less of a threat.
Gemini starts out with increased health, but decreased speed, which is more noticeable if the larger twin is colored. This actually makes Gemini easier, though it takes longer to defeat. As with the Green Champion, there is a slight chance of only the small twin becoming blue, resolving in the older one acting normally, but the smaller twin moving very slowly. Both twins' overall health is increased.
Gemini starts out with two of the smaller twins attached. The large one is capable of destroying rocks. The first twin comes off when the larger third of Gemini is at half health, and the second comes off when the larger third is slain. As long as the large one is alive, all 3 of them slowly regenerate HP. Smaller ones have increased firepower - shoots 3 blood shots per attack.
It's possible to have only one of them being champion variant.
From the artbook, in one of the sketches, Contusion was going to sport a long stitch around his stomach. From another sketch, he is also going to possess a visible belly-button.
Also, one of the sketches of Suture can be seen with a few Z's while sleeping.
Suture was going to possess many teeth when enraged.
Contusion was supposed to sit down when catching his breath for a few seconds after chasing after the character.
Gemini appears in the demo for the original game.
Gemini will occasionally be replaced by Steven after Isaac complete the Basement 25 times.
Gemini is named after the starsign of the same name, represented by the twins.
Their names stem from a cut boss concept (similar to that of Gemini) in Gish, another game developed by Edmund McMillen. According to Edmund, the larger twin is named Contusion due to the dent in his head, which would make the smaller twin Suture.
Gemini may be based on the Twin-to-Twin Transfusion Syndrome (TTTS), which is characterized by joined arteries in a shared placenta between twins. This can result in unequal distribution of nutrition to the twins, leading to a bigger and a smaller twin.
Contusion is the medical name for a bruise, whereas suture is the medical name for a thread used to close gaping wounds, commonly called stitches.
From the original game, Suture used to make a sound after Isaac defeated Contusion.
The Binding of Isaac - Isaac vs Gemini FLAWLESS RUN