Phantom Girl (幻影) was a mysterious boss that appeared for an event in Senran Kagura: New Wave. Her true form and identity were never revealed.
In an event celebrating the Gessen Girls' Academy students being added to the game in May of 2013, this unidentified phantom appeared in the form of each of the five girls. The event described the appearance as a "disturbing shadow" spreading over the school.
As of the conclusion of the event, the identity of the phantom was unknown, and she is not known to have appeared again after this, so more information is hard to come by.
As Senki Shu often served as villains in these events, it is possible that the Phantom Girl was merely an illusion created by Reki, but the Senki Shu did not appear during this event so that is just conjecture.
Phantom Girl was a boss that appeared for an event in Senran Kagura: New Wave. She was fought five times during the event, one battle as each of the Gessen Girls. She is not known to have appeared after that.
Defeating them awarded Justice Points during the event.