Supreme Commander Special Week, you can get her for free in the latest chapter in the main story if you complete certain conditions

laisy said:

Supreme Commander Special Week, you can get her for free in the latest chapter in the main story if you complete certain conditions

Which is to win the Arima Kinen that you are supposed to lose. You can only win it if you have a girl with the 日本一のウマ娘 skill obtained from the Team Sirius card given in the previous chapter.
For reference: https://www.reddit.com/r/UmaMusume/comments/w3g2ku/psa_theres_a_secret_freebie_alt_a_hidden_chapter/

And with unique cut-in animation as well.