Hiveminded Individual

Two or more characters (frequently twins) are in such perfect harmony that they seem almost to be one person with two bodies. They finish each other's sentences, never seem to need to talk to communicate, and may even know what is happening to each other from far away.

In Sci Fi or Fantasy series, the connection may actually be a true shared mind, either with each member contributing to the whole, or the separate bodies being puppets which some central mind controls remotely. It makes sense that usually the first variant is sensitive to losses and avoids overt violence, but in the second, it's only the question of whether lost bodies will be replenished and it's inclined to expand itself. Expect "individuals" in such hives to be considered very killable by everyone else as well. It is not unusual for it to start out as the former and then slip into the latter as a series progresses and the writing staff changes. The Virus is often a Hive Mind (e.g., the Borg) and the Evil Matriarch becomes its Hive Queen.

Hive minds are not known to exist in reality -- hive insects, upon which the idea is based, communicate intentions and commands through scent and body language. The closest approach to them would be the controversial superorganism concept.

Related to, but separate from Synchronization, where each individual experiences what the other does without necessarily being in rapport with each other.