Gemini are a set of conjoined twins (parasitic twins) attached by an umbilical cord.
The large one has two teeth bucked out and is missing an eye with multiple markings.
The larger of the two will walk towards Isaac, trying to deal contact damage. Eventually, he starts charging with increased speed for a 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 become enraged, abandoning ranged attacks in favor of charging at Isaac (if you deliberately focus-fire the small one from the start, it will be killed and you 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.
- Your best option for handling these two is to run around the edges of the room, firing behind you as you 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 you find yourself 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.
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 coloured. 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 will occasionally be replaced by Steven after you complete The Basement 25 times.
- Gemini is named after the starsign by the same name, symbolised by the twins.
- Gemini appears in the demo.
- Gemini are also known as simply The Twins according to the "Unholy Edition" art booklet. Their individual names are given as Suture and Contusion, with the larger one being Contusion (note the sewn-on patch of skin on his head) and the smaller one being Suture.
- Their names stem from a cut boss concept (similar to that of Gemini) of Gish, another game from 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.
|Basement||Larry Jr. • Monstro • The Duke of Flies • Gemini • Steven • Famine|
|Cellar||Widow • Pin • The Duke of Flies • Famine • Gurdy Jr. • The Blighted Ovum|
|The Caves||Fistula • Peep • Gurdy • Chub • C.H.A.D. • Pestilence|
|Catacombs||The Husk • The Hollow • The Blighted Ovum • The Carrion Queen • Peep • Pestilence • The Wretched • Gurdy Jr.|
|The Depths||Loki • Monstro II • Gish • War • Mom|
|Necropolis||Mask of Infamy • Daddy Long Legs • The Carrion Queen • The Bloat • Loki • War • The Wretched • Mom|
|The Womb||Scolex • Blastocyst • The Bloat • Death • Conquest • Daddy Long Legs • Triachnid • Teratoma • Mom's Heart • It Lives|
|Utero||Teratoma • The Bloat • Death • Conquest • Daddy Long Legs • Triachnid • Lokii • Mom's Heart • It Lives|
|Sheol||The Fallen • Satan|
|First to fifth and seventh level||The Fallen • Headless Horseman|