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.
- 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「