|“||Forget infection. Those were made to destroy the world!||”|
–Arakawa, witnessing the raw power of unchecked vectors, driven by pure rage
Vectors are invisible "arms," telekinetic in nature and origin, that most Diclonii appear to possess. They are a Diclonius' primary means of defense and, more commonly, offense against humans. However, it is believed that their real function isn't to kill, but to infect humans with the Diclonius Virus, as pointed out by Kurama.
In the series
Depending on the field of science, a "vector" can be a force with definitive direction (physics) and a means by which a virus is spread (biology). Both of these definitions apply to the vectors used by the Diclonius race, as they have traits of both. In addition to manipulating objects according to the Diclonius' will, they mutate human DNA so that all of a male human's children will be Diclonii from the point of infection onward.
Vectors are telekinetic energy weapons that manifest as invisible arms and are even called "arms" by those who wield them. In the manga, they appear to merely hover around their owner or, in a few instances, lie limply on the ground behind or around them until needed. However, the latter trait is eventually phased out of the manga's representation of them and they appear to only hover later on. In the anime, they appear to come straight out of their owner's back.
Regardless of how they manifest, they are mainly used as weapons, with which the Diclonius can move, bludgeon, or, most commonly, cut up any offending person or object in their way. They can destroy or repel most materials and projectiles, with such notable exceptions being the specialized prison doors at the Diclonius Research Institute and the fifty-caliber bullets used in such weapons in the AW50F and the Desert Eagle.
Depending on the individual Diclonius' power, they can be strong enough to bring down military helicopters and even sink islands, yet can perform extremely delicate tasks as well, such as killing humans by manipulating their blood vessels, or, in Lucy's case, piecing together damaged cells to repair them. They can be used as shields, such as when Mariko protected Kurama and Nana from a missile strike and when Alicia and Barbara used theirs to save Nousou from being crushed by a helicopter. They can be used for any mundane task, such as cleaning. Shown by individual Diclonii and the Vector Attack Craft alike, they can be used to transport their owner as if they were regular limbs or by launching them through the air.
Vectors are stated to be made up of molecules, and the vibration frequency of these molecules determines their abilities:
- Low: can go through objects without causing physical interference
- Medium: lift and throw objects, disrupt blood vessels
- High: slash through most material of low density, carry liquid on surface
- Extremely high: visible, explosively powerful
The number of vectors varies from individual to individual, as does the maximum length at which they can stretch them. Mariko possessed the most vectors of any Diclonii, 50 in the manga and 26 in the anime, and had a range of 11 meters. At the end of the manga, Lucy's vectors, typically numbering at 14 (manga) or 4 (anime) and 2 meters long, numbered around 19 or 20 and had a worldwide range and were powerful enough to sink whole islands and potentially destroy the entire world. Overusing their vectors can cause a Diclonius' body to deteriorate due to entropy and even eventually melt.
Despite their enormous power, vectors can be disabled at least two ways: by inhibiting the egg-sized pineal gland in the frontal lobe of their brain, and by breaking one or both of their horns. Nana and numerous others associated with the Diclonius Research Institute utilized both methods to incapacitate Lucy and other Diclonii. In Nana's case, she used her vectors to pierce the pineal gland to disable Lucy and Mariko's vectors for a few hours at a time. The Diclonius Research Institute used mind control devices in different Diclonii's foreheads to inhibit their vectors and their more volatile personalities. On two separate occasions, Lucy's horns were broken (first by the explosion of Mariko's bombs and the second time by Cynthia's vectors and the Agent shooting her). Both times, she reverted to her Nyuu personality, and it's been stated that if a Diclonius loses one or both horns, they can become comatose or even die in addition to losing their powers. In the anime, vectors can be grappled with by other vectors to stop them from reaching a target, which can be best seen during Lucy and Mariko's showdown in Episode 13. This does not appear to be the case in the manga, where they can be felt by those they injure and perhaps even heard if one considers the sound of air rushing around them as they move, and otherwise, they are unable to be seen or acted upon by other forces save for rare exceptions.
- In the anime, it seems as though vectors can be seen if covered in blood, as Lucy's vectors leave bloody hand prints on at least three occasions, meaning they can hold onto and even carry blood molecules. However, this could possibly just be an artistic effect, as the bloody hand prints only show up in the first episode and promotional artwork and don't actually appear onscreen past the first episode.
- When launching a Diclonius into the air, vectors leave hand-shaped imprints in the ground. They also can leave hand imprints in other things with enough force, such as the hand marks left around Lucy's room in the orphanage during her childhood flashback.
- Diclonii appear to be able to sense each other's vectors to some degree. Most Humans cannot see them. Kouta's little sister Kanae claimed to have seen Lucy's vectors at the festival, but it remains unclear how.