The Burn Smash is a Smash monster created from the girlfriend of Ryuga Banjou, Kasumi Ogura, and an antagonist in episode 2 of Kamen Rider Build.
After Ryuga escaped from prison, Kasumi was abducted by Night Rogue and Faust and experimented on to be turned into the Burn Smash. It then attacked Ryuga and Sento when they came to rescue her. As Build was fighting the Smash, Night Rogue informed the Rider and Ryuga that Kasumi had a "Hazard Level 1" and thus could not return to her original human form.
Build attempted to finish off the Smash, but was interrupted by Ryuga. The Smash then tried to attack Ryuga, but Kasumi's consciousness managed to hold it back. Realizing it was too late to save Kasumi, Ryuga pleaded with Build to end Kasumi's suffering. Using the GorillaMod form, Build temporarily separated Kasumi and the Burn Smash, allowing her and Ryuga to say their final words to each other. Kasumi's consciousness then faded, and Build extracted the essence of the Burn Smash. He then purified the essence into the Dragon Fullbottle and gave it to Ryuga, allowing him to carry a memory of Kasumi.