Y/N L/N was adopted to the Ichinose Family with an unknown origin and he is a brother to Houtaro Ichinose both of them are students at Furasu High School. One day, they discovers a secret school for alchemists in training called the Alchemy Academy beneath the school's premises and meets alchemist Fuga Kudo, who entrusts Houtaro with the Gotchard Driver, allowing him to transform into Kamen Rider Gotchard and entrusts Y/N with the Legend Driver allowing him to become the Rider of all Legend Riders Kamen Rider Legend. He tasks Y/N and Houtaro with recollecting the Chemy and Legend Kamen Riders Chemy, artificial lifeforms created using Alchemy that were released in the wild 10 years ago and, if left unchecked, can transform into monsters called Malgam after being corrupted by human malice Assisting Y/N and Houtaro in his task to retrieve the Chemy is Fuga's daughter Rinne, a student at the Alchemy Academy. Supana Kurogane, a graduate from the academy who views the Chemy as nothing more than tools and transforms into the warrior Valvarad, was also tasked with retrieving the Chemy and Legend Rider Chemy and is a rival to Y/N and Houtaro who wants to befriend them. They are opposed by the Three Dark Sisters: Atropos, Clotho and Lachesis, who intend to use the Chemy and Legend Rider Chemy for their own purposes.

In an alternative reality, a young man named Shoma Kashimiya was running away from Stomach Inc. Using the Door Space, Shouma managed to escaped to Human World. However, the Human World he arrived at was not a good one. There is a monster called, Noise that threatening humanity. Shoma also awaken his power in Human World after eating the snacks. He then decided to using that power to protect human from the Noises and Stomach Inc. ~•~•~•~•~ Disclaimer: This is just a fanfiction story. I didn't own the Kamen Rider Gavv or Senki Zesshou Symphogear either. Some pictures are made using AI. I hope you enjoy this fanfiction crossover story.