When Kaede was still a child at the orphanage near Kamakura, she began to feel her future vectors begin to manifest themselves, and also began to hear a voice urging her to take vengeance on those who delighted in tormenting and even betraying her. After the bullies killed her puppy and were killed by her in return, young Kaede began to almost unconsciously obey the murderous will of this voice, which had her killing whole families for a place to stay even as she befriended Kouta. After Kouta's seeming betrayal of her trust, Kaede surrendered entirely to the will of this voice, resulting in the murder of Kouta's sister and Father. Over the next five years, Kaede engaged in a stealth campaign, using her vectors to cause fatal heart problems in some Kamakura residents while infecting the fertile young males with the Diclonius virus, causing their children to be born with horns and vectors. The voice's hold on her during this period is at best unclear. It is possible her stealth campaign was part of its plan, or that it wanted a more confrontational approach. In any event, the Voice may well have been her only companion mentally during her three-year imprisonment.
After her escape, one of Kaede/Lucy's goals was to find Kouta so that she could apologize to him for her murder of his family. Ironically, they both met again while amnesiac, and Lucy had become the child-like Nyu, who had no vectors and did not heed the DNA Voice at all. During Lucy's first reemergence, she seemed set to kill her hosts at Maple House until she saw that one of them was Kouta.
Many times, Lucy's own freely chosen actions were hard to distinguish from those the voice might urge her to. It only seemed to speak up directly when she did something that ran contrary to what most doubtless saw as Lucy's own vicious nature. Her cruel dismemberment of Nana occurred after some warning was given to the younger Diclonius prior to the battle. Her eventual murders of Mariko and Shirakawa can also be taken as part of her ongoing, highly personal and continually lethal feud with Kurama. Her attempt to kill Mayu when the young girl's memories of Nana's fight with Lucy returned were meant to keep Mayu from telling Kouta what she knew. Since the DNA Voice did not want her relationship with Kouta to continue, it in theory might want Mayu alive to possibly expose Lucy/Nyu altogether. Lucy once told Kouta that the voice could not give her commands, only advice, yet not long after this the Voice actively seized control of her body to stop Kurama from killing her.
Lucy openly defied the voice on two notable occasions after a long period of almost blindly taking its advice. Once came five years after her murder of Kouta's family, when she met and befriended Aiko Takada. The other came at the very end of Lucy's life, when she chose to heal Kouta rather than destroy the world, even though this also meant the destruction of her body's integrity. In other instances, the struggle was more muddled and the outcome less certain either way.
Nana heard a voice like this one, and this arguably was in fact the same voice, when, despondent over leaving her 'Papa' Kurama and taunted by Bando as having no place in this world, Nana briefly sought to kill him and everyone else.
Later, after the viable Mariko clone named Barbara had been freed from an infantile state of control by Nousou, whom she then killed, she pursued Nana, whom she saw a traitor for living happily among humans. During the confrontation, she chastised Nana, asking if she didn't hear the voice that was urging them to kill all humans.
The existence of the voice is a debatable point on a few levels. While the actions of many Diclonii towards humans would seem to indicate an inborn drive to kill, other factors mitigate the starkness of these seemingly automatic killing sprees. One is the age that most Silpelits gain their vectors at, usually being about three, a volatile time for any small child. Another factor would be the treatment by their families of these fast-aging children as they grow, ranging from abandonment to a wary acceptance of their horns, again with their vectors emerging at a difficult age where discipline is not yet learned. It is arguable that these children need no drive to kill, since a lack of understanding of their abilities plus potential mistreatment make them apt to lash out in any event.
Yet Lucy certainly felt this drive and urge, at some times more strongly than others. But the girl Kaede as she suffered trauma and betrayals real and imagined could easily have been suffering a schizophrenic breakdown as she heard this voice, which could have been a true 'DNA Voice' or could have been a mental state she placed her incredible pain in. The fact that at least two other Diclonii seemed to hear this voice can be explained by a DNA link, a side effect of Lucy's 'Queen' status broadcasting her pain and rage out to others like her, or the fact that all three had suffered at the hands of humans and wished revenge on some level for it. The voice seems to come out at the times of greatest alienation, literally a feeling that they have no place in the world.
To young Kaede, this feeling is the final acceptance of something she has fought against her whole life. To Nana, it is a shock and surprise to a girl who thought of Kurama as defining her place in the world, but she regains and even supplements this feeling when she becomes part of Maple House, only moving to violence when her new home and family and Kurama are threatened. To Barbara, the alienation is almost reversed; She is happy not to be human, feels strongly that they and Diclonii who hold with them must be destroyed, and loudly shrugs off the idea that she should have some connection with her genetic father, Kurama. That said, Kurama is able to kill Barbara with a gunshot he is almost certain she knew was coming, so it perhaps becomes the undoing of her accepted alienation that throws her off. It might also be worth noting that, at this same time, Lucy, now a captive of Chief Kakuzawa, firmly rejects the idea that she has no place in the world, wanting to get back to Maple House. If Lucy was the receptacle for the DNA Voice, whether it was a true genetic instinct or her own embodied pain, it could be that her rejection also affected Barbara. When Nana nearly gave in while facing Bando, Lucy had recently forced herself to become Nyu again rather than admit her murder of Kouta's family to him, perhaps leaving the voice needing somewhere to go. Again, since each of these three girls had known huge suffering and pain, each voice heard could have been merely the result of that, with Lucy's the most developed due to her having lived the longest and fullest life compared to others of her kind.
Lucy had felt that she merely took advice and not commands from the voice. Kouta, when finally told of all this after his childhood memories came back, harshly dismissed all talk of a voice as something humans struggle with every day. Yet very soon after this conversation, the Voice seized control of Lucy and attacked Kurama when Lucy refused to defend herself, this to keep a promise made to Kouta. After both the Lucy and Nyu personas had fled their dying body, Nana felt a persona that was neither or them take over the melted remains, though it apparently never spoke to her. In rage and agony, the Voice killed the last squad of soldiers sent after them, and halted when the pain of the disintegrating body proved too much, but still intent on destroying the world before it died. Nyu and Lucy's spirits protected Kouta and their friends from the Voice so that Kouta could kill their body once and for all, but to the Voice's shock, he proved unable to do it. The Voice at last found the pain so great, it pleaded with Kouta to end the body's life so it too could die, and out of mercy, love and revenge, he complied.
In the manga series' finale, Kouta offers up an account of the Diclonii War that emerged from the plans of the late Chief Kakuzawa, but no mention is made of an inborn voice driving the Diclonii. If the Voice ever existed outside of an individual level, then either it moved on to these girls as they rose up, or it died with Lucy's body, regardless of whether it was true genetic instinct or her own broadcasted pain. That raises the unanswerable question of whether these Silpelits during the war ran on this instinct or were wilder and more unfocused in their attacks because they had no guidance.
When seen inside Lucy's mind, the voice could be a nude or bandaged-up child, and in the manga, seemed to have its own separate thought patterns. It seems to have played no part in the possible reincarnations of Lucy and Nyu at the story's very end.