I will genuinely cry if it turns out Riri had something to do with the demon lord's death. I did NOT like that hand trembling here.
Purely speculative, but this series was based on a one-shot titled "It's Too Early for the Demon Lord" that the author wrote in 2020. There are some significant differences though, such as Ooma being a 4-year old boy and Riri being one of the preschool teachers. The characters were pretty different though, as Ooma was more assertive and haughty and the romantic tension between the two was significantly less.
Anyway, in that version Riri loses something important and Ooma recruits the other kids and some animals to go and find it. At the end of chapter, Ooma finds the important thing, a ring which he had previously given her in the other world, and essentially proposes to Riri. On the final page, Ooma wonders how the ring even got to this world in the first place, and unbeknownst to him, Riri is shown transforming from her original form back into the preschool teacher form with a devil tail, indicating that she didn't actually reincarnate into this world but found a way to travel to the world where Ooma was residing.
That might be her secret in this version too, but it might be something completely different since both versions are very different.