S'yne's Sister/Chronology

Volume 17
During the heroes' fight against Miyaji to save Kizuna, S'yne's sister had appeared. She didn't help out significantly during the fight but decided to "have fun" with her sister instead. During the battle, she explains to Naofumi and the heroes how the technology to suppress their Holy Weapons had worked and how to liberate Vassal Weapons which have been suppressed. After Miyaji, Mald and Malty had been defeated, S'yne's sister picked up the survivors and started to leave. At the resistance of the heroes, she knocked them all down in a single blow. She also reveals that the vanguards of the waves that they have been fighting against are only "low-level thugs." She then leaves without a trace.

Volume 18
S'yne's sister later appears among a crowd of volunteers for L'Arc's forces. She comes with Lesty and three vanguards of the waves under her control. After some conversation, her forces and the heroes fight. During the fight, she also tested out the Artificial Behemoth against the heroes while she stood back and watched. Upon the Demon Dragon's request, most of the heroes worked together with her and they were able to kill Lesty and the main vanguard of the wave - "Enemy of the Week." S'yne's sister teleported away before the attack was landed. Now that the heroes had a single enemy remaining - the Artificial Behemoth - they were able to work together to defeat it. With its defeat, S'yne's sister appeared to take the corrupted legendary weapon inside. The heroes try to take her on again but are quickly defeated. She then uses a movement skill to leave.