In fact, she was the child that was to be born in the future to Airi Nogami and a mysterious man (Yuto Sakurai), and she was the key to the fork in the road that Kai was after. Her existence was hidden by the power of the Zeronos card, causing a delay in her birth time, and when a new route to the future appeared, her body took on the form of a child (Kohana).
The card that Yuto Sakurai inserts into the Zeronos belt when he transforms into Kamen Rider Zeronos.
When inserted with the green face facing outward, the card transforms into Zeronos Altair form, and when inserted with the yellow backside facing outward, the card transforms into Zeronos Vega form.
Each transformation causes the card to disappear, and each card consumed causes the loss of memories about Yuto held by others.
Therefore, the danger is that all the cards will be consumed and all people will lose their memories of Yuto (and Yuto himself will disappear).
However, the card itself has the ability to store memories, so if Yuto disappears due to historical change, it is possible to revive him, although it will take some time.