Resized to 85% of original (view original)
Artist's commentary
Oho, I see🤔
Reference to this tweet from the Assault Lily: Last Bullet official account hinting at what was, at the time, an upcoming costume.
The costume is actually Andou Tazusa's (but, at the time the art was drawn, was already highly unlikely to be Yuyu due to the lack of visible long hair.)