Iduna is a young woman with long blonde hair and blue eyes. She wears a white shirt with half-length sleeves and pink overall shorts with a blue pocket, which often holds her tools. She wears pink and blue matching shoes. Iduna is often seen wearing a welding helmet.
Ever since she was 5 years old, Iduna had already shown a sign of her talent in inventing. She has always liked creating various things not only for fun, but also to make her parents happy and be praised by the adults around her. This talent allowed her to join C3 at the age of 16.
Iduna is very laid back and even ditsy at times. She enjoys creating new inventions and getting feedback on them. She is very open with her feelings and will say what is on her mind. Iduna is very intelligent, as she is always creating different gadgets and improving existing technology. She can be forceful at times when she becomes passionate about upgrading things, to the point where it scares others off, such as Mikuni, due to her repeated attempts to turn Abel, Mikuni's doll, into a robot. Despite this, she is very light-hearted and open to others. Unlike Shuuhei, she has no aversion to vampires, believing not all of them are as bad as he thinks. Iduna has a good relationship with her coworkers, simply hanging out with them or often suggesting that they play games where the loser buys everyone lunch.
Iduna makes her first appearance in chapter 13 when she uses a stun gun on Mahiru. She knocks him completely unconscious and regards this as a mistake, supposing she set the gun too high. She is with C3's Robot Girl, who is assumed to be made by Iduna.
While Mahiru is with Shuuhei, Iduna, along with an unnamed man wearing three pairs of glasses, look after Kuro. Iduna sits on a couch next to Glasses Man holding noodles. She calls herself silly because she waited five minutes for her noodles and not the correct time, three minutes. While trying to interrogate Kuro, Glasses Man tells him that Iduna over-stunned them and then tells her to apologize. Izuna smiles and says no one understands her.
When Mahiru and Kuro stumble into a room at C3, Iduna sits in a control room watching them. Shuuhei barges in looking for them and asks Iduna if they escaped.
Iduna then asks Shuuhei why Mahiru can't just leave, telling him not to be upset with her for asking. Shuuhei is confused and Iduna remarks that Sloth is different now. Mikuni then enters and Iduna tries to turn Abel into a robot. Mikuni refers to her as Evil Welding Girl and tells her to stop lusting after Abel. Shuuhei then locates Mahiru and Kuro and goes after them. Iduna decides to join him. Here she pets Kuro, who is in cat form, and calls him cute. Before Mahiru leaves, Iduna introduces herself as Shuuhei’s coworker and tells him that Shuuhei’s father was in C3 until a vampire killed him. She says that Shuuhei still holds a grudge, but thanks to him, he likes vampires a little more than the day before. She then reveals she was the one who stun-gunned Mahiru and asks him for feedback.
When Mahiru and his friends are taken under C3's custody, she shows Mahiru, Tsurugi, Yumikage, and Junichiro her new creations, one of them being a gun that shoots artificial sunlight that turns Kuro into his cat form. She occasionally shows support of the idea to peacefully communicate with the vampires. During the prison break, she lends her ID Card to Mahiru so he can use it to free The Mother while she evacuates the other workers until Touma orders her to evacuate as well. On her way, she meets Ray, Gil, Lawless, and Licht. Lawless returns her ID Card and thanks her for lending it as it helped them freeing The Mother. This earned her Gil and Ray's gratitude and trust.
Shortly after, they are confronted by Touma who is holding Tsurugi. Despite her attempt to convince him it's not necessary to fight, Touma attacks her, but she is saved by Gil and Ray. When Touma shot Ray with the gun she creates, she quickly tells Gil to cut off Ray's right arm that slowly disintegrating, forcing Gil to do as she told him to. Moving Ray away with Gil while Lawless and Licht fights Touma, Iduna guiltily admits that she is the one who created the weapon. Kuro arrives with Yumikage right before Touma could kill Lawless and Licht. Hearing what happened to The Mother, Gil and Ray pleads to her to save The Mother. She determinedly promised to leave everything to her and leaves searching for The Mother. While looking for The Mother, she recalls her past and her regret for inadvertently motivating Shuuhei to become a killer by creating weapons to kill vampires. Finding the fourth servamp greatly weakened in her wolf form, Iduna tearfully asks her to form a temporary contract with The Mother so she can stop Shuuhei from becoming a killer. As The Mother comforts her, Iduna reveals her name and gives The Mother her new name, "Freya".
Iduna is Shuuhei’s childhood friend, therefore, the two of them get along. She knows about his fathers death and was by his side to support him through that difficult experience. Iduna knows him well enough to realize that Mahiru made him like vampires more than he did before. Iduna doesn’t want Shuuhei to become a killer and wishes to prevent him from killing Shamrock and avenging his father. Iduna has a nickname for Shuuhei, calling him "Teriyaki", which Shuuhei usually ignores or corrects. It is hinted that she may have romantic feelings for him.
- "Strange...Sloth seems different now." (Chapter 15)
- "I'll turn Abel into a robot!" (Chapter 15)
- "I'm Teriyaki's coworker...er, Tsuyuki-kun, I mean...!" (Chapter 15)
- "A world of hate...is a sad, sad place." (Chapter 15)
- Before her actual name is revealed, the fandom dubbed her as Tinker due to her occupation and hobbies.
- She does not appear in the anime.