|This article is about a/an monster in Kamen Rider Zero-One.|
Seine was a Humagear ordered by Seiji Tazawa, basing her appearance off of his deceased daughter, Sumire Tazawa. Seiji has made an application for Seine to become a Voice Actor-Type Humagear for Choichiro Ishizumi's anime adaptation of his work.
After advancing in the audition stage, A.I.M.S.' Isamu Fuwa was dispatched to interrogate and arrest Seiji and dispose Seine, due to violating Article 6 of the A.I. Law, which prohibit creation of Humagears in the likeness of real human beings without the permission of the person being copied upon. Aruto, who was empathic on Seiji's predicament, tried stopping Fuwa, when Jin arrived with a new Onycho Magia with the intent of stealing a Progrisekey. This time Jin succeeded in stealing Aruto's Flying Falcon Key. Within the commotion, Seiji took the chance to get Seine and himself away as possible, but was stopped short by one of the newly-converted Trilobite Magias, damaging Seine's left earpiece. Noticing the Humagear not targeted by the transformation coil from Onycho, Jin was about to put a Zetsumeriser on her, when he noticed Seiji protecting her, claiming her to be his child, which confused Jin for a while. Seiji then took this chance to get himself and Seine away to escape.
Back in Seiji's office, Seiji rebooted Seine, revealing a recording of Sumire's voice as part of the start-up audio, which Seiji frantically tried to cover-up unsuccessfully. Knowing that Seine would be taken away from him after all that, he pleaded to have Seine in his custody for another day if possible. Aruto assured Seiji that he was still assessing the situation since he has gone through a similar loss with his father, and was pondering on what would be the best solution. Fuwa then arrives and was about to arrest Seiji, but was stopped again by Aruto, asking for more time. During the live audition, Seine started showing human-like personalities, and was speaking lines that were not in the original script she was given, to which Seiji apologized, saying that she recently got damaged. From there, Jin appeared with the intent of using Seine to help him get another Progrisekey, claiming that they could be friends since they both were someone else’s children. After installing the Zetsumeriser on Seine, Seiji unsuccessfully tried to protect her. After Aruto angrily explained to Jin why a parent would want to protect their child, a confused Jin tried to forcefully tear off the belt out of Seine to no avail. Seine then activated her Gaeru Zetsumerisekey and transformed to battle Zero-One.
After the battle, during which Kamen Rider Valkyrie destroyed the Gaeru Magia, Izu reported to Aruto that Hiden Intelligence appropriated a replacement A.I. with Seine's/Sumire's data to Seiji instead of a replacement Humagear, due to his violation, which nevertheless appeased Seiji.
Sumire Tazawa (多澤すみれ) is Seine's physical template, and Seiji Tazawa's biological daughter. She was an aspiring talent, and was said to have had a major break in her career when she started to fall ill. She died three years before Seiji purchased Seine, who was made to resemble Sumire.
- ―Transformation announcement[src]
- Height: 192.9 cm
- Weight: 109.2 kg
Seine Kanasawa transforms into the Gaeru Magia (カエルマギア Gaerumagia) by using the Gaeru Magia Zetsumerisekey in the Zetsumeriser. Its large frog head can open its mouth, revealing its true head inside. 
- Kogael Bomber (コガエルボマー Kogaeru bomā): Small frog-shaped bombs that can be rapidly launched from the Gaeru Magia's "mouth" and explode upon impact.
- Humagear Hacking: Using a method similar to transforming into a Magia, the Gaeru Magia can corrupt other Humagear and transform them into Trilobite Magia by piercing them with tendrils.
- Swimming: The Gaeru Magia is adept at swimming.
Behind the Scenes
The Gaeru Magia's name is derived from Ibukurokomorigaeru (イブクロコモリガエル), the Japanese name for the gastric-brooding frog (Rheobatrachus): a genus of ground-dwelling frogs that temporarily went extinct in the 1980s and are known for raising their young inside their stomachs.
- The Gaeru Magia is the first Frog-themed monster since the Frog Fangire.
- The Gaeru Magia is the first Magia whose name is not derived from the scientific taxonomy of the extinct animal it is based on.
- Her name is play on voice actress Kana Hanazawa, who is known for voice acting leading roles in many of anime series.
- Gaeru Magia resembles Ene-Tan from Tokumei Sentai Go-Busters.
- The Gaeru Magia is the first female Magia that is not a footsoldier.
- Her form as the Gaeru Magia, alongside being designed as a voice actress may also be a reference to the phrase “A Frog in the Throat”.
- The Gaeru Magia is the first Magia who is based on a living animal.