Nasiens is a young boy with short light brown hair and yellow eyes.
Nasiens seems to be a calm and serene boy, who can also be scary at times, and rarely makes his true feelings known. Although with Percival, Donny and Sin, he has managed to have enough trust to show them his emotions without hiding anything. However, Nasiens still keeps some secrets, as when Anghalhad perceived that he was hiding something.
Nasiens has a great interest in plants, wanting to be a herbalist like his adoptive grandfather Ordo was. He gets very excited when making new discoveries, liking to experiment with everything around him.
Nasiens can be very sentimental, but being isolated for so many years, he does not know how to show it; an example of this would be when he could not find the words to tell Percival that he considered him his first friend, and ended up saying "guinea pig".
Nasiens was abandoned as a baby in Echo Gorge and was found and adopted by Ordo. Nasiens grew up studying to be an apothecary like his grandfather, Ordo, until one day Ordo disappeared and soon after, a strange disease began to eat the life force of the plants and animals of Echo Gorge.
Nasiens started conducting experiments to create a cure for Echo Gorge, and as a side effect of some of his failed drugs, the animals and plants became monstrous and violent. This led to the local fairies to mistakenly believe that Nasiens was trying to destroy Echo Gorge.
In his lair, Nasiens talks to a strange fairy he is holding hostage. The fairy says that he just got lost and ended up in the Echo Gorge, to which Nasiens decides to turn him into his guinea pig. There, Percival breaks into his lair and manages to free the fairy. However, Nasiens sleeps him with belladona leaves with the intention of turning him into his guinea pig instead.
When Percival wakes up tied up and almost naked, Nasiens is surprised as the belladonna should have kept him asleep for ten hours. After mentioning the magical nature of Percival's helmet and cape, Nasiens injects him with a strange drug that makes his hair grow rapidly, to which Nasiens concludes that he could use it to accelerate the growth of plants and animals. Percival claims what he does to him and what he did to the fairy, but Nasiens explains that the fairy was the one who invaded his home and attacked him first. Nasiens says that the speed with which he responds to his treatments, Percival is the best of all guinea pigs. Percival marks the way to talk about him and how blood falls from his mouth, which Nasiens explains is a habit of biting his lip when he is excited.
While preparing a drug, Percival asks Nasiens why he is doing all this, to which Nasiens refuses to reply. There, Percival manages to free himself and recover the helmet from him, which surprises Nasiens and leads him to take up his weapons and infuse them with magic to fight Percival. Percival tells him that he is willing to help him if he needs it, but Nasiens says that all he needs are guinea pigs to finish the drug on him. When the helmet flies off the staff head due to his hair, it hits Nasiens, causing him to fall onto his workbench and Percival to catch the substance he was developing. Nasiens demands that he return it, but Percival drinks it since Nasiens wanted to test it on someone anyway.
Despite suffering at first, Percival ends up energized after drinking the substance. Nasiens asks him why he drank something so dangerous, to which Percival tells him that it was to help him since he created that substance to save Echo Gorge. Seeing his fruitful work, Nasiens drinks what is left of his pot. When the gorge fairies attack Nasiens believing him responsible for the disaster in his home, Percival defends him by revealing his true intentions. Nasiens then uses his magic to recreate his drug and spread it throughout the gorge, reviving it completely.
Once the misunderstandings are cleared up, everyone there is attacked by a monster that spreads poison over the place crying out for the destruction of Echo Gorge. The monster is revealed to be Ordo, acting under the command of the fairy Nasiens had captured. Nasiens tries to reason with Ordo to no avail, being saved from him by Dolores who ends being hitted by Ordo's poisonous breath. While worrying for Dolores, Nasiens is once again saved from Ordo this time by Percival. When Nasiens asks what he is supossed to do, Percival says that they will bring Ordo back together.
The fairy is later revealed to be a minion controlled by a Holy Knight named Talisker. Nasiens demands that he tell him why he did that to Ordo, to which Talisker says only if he manages to hit him. Nasiens immediately prepares to fight by infusing his weapons with poison, rejecting Percival's help when he is unable to invoke his magic. Talisker then summons a hailstorm from which Nasiens is saved by Percival. However, Nasiens takes a heavy hit from Ordo. Although Percival manages to get in the way and tries to bring him to his senses, Nasiens thinks it is too late. However, a trace of consciousness shows up on Ordo, so Percival protects him from another hailstorm resulting in severe injury.
When Percival manages to confront Talisker by unleashing his magical power, Nasiens hears the story of his conflict with Ordo and how it turned him into a monster. After Percival defeats Talisker and destroys the staff that converted Ordo, the latter returns to normal to the joy of Nasiens.
When Percival, Donny and Sin were about to leave, Nasiens catches up with them saying that he is going with them. When questioned about it, Nasiens tells them that Ordo asked him to travel the world to become a better apotechary than him. Nasiens tries to tell Percival that he is the first friend he has, but ends up saying that he is his first guinea pig. Even so, Percival accepts him into the group to the delight of Nasiens. Saying they need transportation to get to Liones, Nasiens suggests going to Sistana to get it.
When the group stops to eat, Nasiens experiments with the effects of his poison on Percival's mini golems. After enjoying the food that Percival cooks for them, Nasiens asks him about his grandfather's magical objects and expresses his doubts that they can reach Camelot, a destroyed kingdom, from Liones despite Percival insisting that Sin would not lie to them. After discussing the events that Percival, Donny and Sin went through before meeting Nasiens, the topic of Sin's Incantation Orb comes up for discussion and Nasiens explains what they are and how the magician Merlin created them. He also talks about how Merlin has a book with all the poisons in the world and how he wishes he could see it.
Upon reaching Sistana, a mini golem leads them to find a piece of the Coffin of Eternal Darkness buried. Nasiens shows her interest in her despite not knowing what it is. A girl suddenly arrive and warns them to leave it and to kill them if they intend to give it to "that man".
Although Donny tries to explain that they happened to find it, Anghalhad pulls out his rapier and attacks him, managing to dodge and knock him down quickly. Nasiens says that they have no intention of fighting, but she faces him anyway, managing to overcome him in an exchange of swords. She then demands that Percival give her the piece and to her surprise, the boy kindly gives it to her. A maid appears telling Anghalhad that she must return for the dance that is about to begin. Anghalhad rushes to return the Coffin's piece to Percival demanding that he hide it and not let anyone see it, especially Ironside. She then leaves with her maid.
At night, the four of them stay at an inn. Nasiens explains to them that the girl is the daughter of the Lord of Sistana whom he saw several times during his visits with Ordo. They then spy on Ironside's speech to the townspeople, revealing that he owns the other pieces of the Coffin. Sin reveals to them that Ironside plans to kill everyone in town to activate the Coffin magic. Sin suggests that they escape with the missing piece, but Percival sets out to save Anghalhad when she interrupts the speech. Nasiens, Sin, and Donny run to the inn, but discover the maiden stealing the room, actually being Ironside's familiar in disguise. After meeting with Percival and Anghalhad, Nasiens decides to help them stop Ironside.
During the fight that breaks out with Ironside, Nasiens enchants henbane in Anghalhad's rapier, causing dizziness and shock at Ironside. Ironside unleashes his Belfest Margot, seriously wounding everyone, who is saved thanks to Percival's mini golems who heal their wounds. Ironside, however, captures Percival. Ironside begins to beat up Percival and finally decides to kill him by driving his sword into his heart. Nasiens recovers Percival's body and desperately tries to help him with his drug, but it is useless since Percival is physically dead.
Ironside then intends to continue with the Coffin's ritual, when a breeze of magic leaves Percival's body. Ironside destroys everything to finish it off, being this protected by Nasiens. Ironside points out that he was saved thanks to the fact that his poison affected the precision of his magic and warns him to leave if he does not want to die, attacking him again at his refusal. Percival's magic multiplies more and more, leading Ironside to use his Ragna Claw. However, a revived Percival saves everyone with his magical power, which is revealed to be the power of Hope.
Ironside launches his attacks at him, forcing Percival to fly all over the place to evade them. In that, Donny uses his telekinetic powers to levitate the Coffin away from Ironside, allowing Percival to re-divide it. Ironside claims they haven't stopped him as he just has to kill them and reassemble the Coffin and gather more sacrifices from Britannia. However, another Holy Knight, Mortlach, appears stopping him from continuing the fight and reminds him that whoever destroyed the monsters he summoned must be someone as strong as the Seven Deadly Sins, so in the face of such a threat, it is best for them to retreat.
As everyone twists after the hard battle, Nasiens comments on Percival's cape and helmet that were transformed during the fight. Percival then touches the wound that Ironside left on his back causing him a lot of pain.
The next day, Carden fires them thanking them for what they did. Anghalhad then appears and Percival invites her to join the group.
- Mix Venom「
調毒, Mikusu Benomu」: This power enables Nasiens to ingest any kind of poison and "teach" its composition to his body, letting him create his own mix.
Nasiens dual-wields the dagger Henbane「