The Twins have the power of Materialization (実現, Materialization). They focus this power by forming bullets created by the two thinking simultaneously and then shooting it through an identical pair of guns. The bullets' color denotes their effect. The Blue Bomb (青ボム, Blue Bomb) freezes anything it hits and the Red Bomb (赤ボム, Red Bomb) creates a large fireball. The White Bomb (白ボム, White Bomb) destroys anything in its path. The Purple Bomb (紫ボム, Purple Bomb), or the Deceiving Spectacles, creates a mask-like cover of paint over the eyes of the enemies and causes them to see any illusion that the twins want. The Green Bomb (緑ボム, Green Bomb) traps an opponent in a slime-filled bubble. Jasdevi's Hate (ジャスデビの怨念, Jasdevi's Hate) is a technique that does not seem to require a bullet. It materializes a monster that seems to be made of numerous dead human bodies. The monster moves according to Jasdevi's will. Their Materialization skills also allow them to create numerous objects, such a floating, bubble used to trap an opponent, an illusion of the Earl that is derived from a past memory, and, as Jasdevi, an iron maiden used to trap Krory. By shooting one another, David and Jasdero can merge together and become Jasdevi. They no longer rely on the guns to focus their abilities, instead using bare hands. Jasdevi's long hair also becomes a weapon and gains new strength from the desire to create the "the strongest imagined body."
Jasdevi first appears in Edo after failing to kill General Cross as directed multiple times. Although they met with the other Noah and the Earl, they were sent off to kill Cross once again. They reappear upon the Ark following Skin's death, after yet another failure to kill Cross. It is revealed that Cross constantly brushes his debts off and onto them. In a library-like room, they face the Exorcist party, and merely toy with them for a while. Eventually, the twins hit the group with their Purple bullet, causing the party to lose their key and sight of the Noah. However, Krory is able to locate them. After capturing Lenalee and unleashing Jasdevi's Hate and an illusionary Earl, Lavi finds the key, breaking the illusion. The twins become enraged because they are referred to as kids and the merge, becoming Jasdevi. Krory faces them alone, sending the rest of the Exorcists to the next room. Jasdevi has the upper hand for most of the battle, eventually locking Krory in an iron maiden created through their abilities. But Krory, using a new technique, exhausts Jasdevi. They split back into David and Jasdero before falling through the doorway before the room crumbles. This is the last time they have been seen so far.