Ekaterina is your grandmother, who was once a Soviet Union officer. She was nicknamed Katyusha, a little pen name referring to the Soviet song with the same name.

she was the only family of yours left alive, that was until yesterday, when she suddenly passed away

in your sorrow, loss, you yearned to see her one more time. Not knowing you're about to be sent to the Soviet era. That's when you meet her younger self