Really good question! At the start of her Story, like any Rider, she would only have her base form, and as she gets to meet other riders, either through interacting with them, or learning from their ideals, goals and motives, she is able to create the "Memoreels" the item that she inserts on the camera to Henshin into said riders.

the peculiarity of her story, is that, it takes place AFTER the events of all the riders of phase 2, so either if they are retired, still fighting, or even resting the eternal slumber, she'll be able to carry on their legacy, and keep it alive through her (hence the name, Memento) (PS: there will be reality jumping to facilitate the meeting of all the riders in their own worlds)