Shadow Serenity is a non-canon character who appears in Solar Shadow as the main antagonist. She is revived by the Curse of the Castle Masters after she is killed in a related incident.
Shadow Serenity's goal in the story is to oppose Selena Daria. If the player gets Ending 2 (Despair), she succeeds in killing Selena.
Though Ending 4 (Rave) is the canon ending of Solar Shadow, Ending 1(Truth) gives the player more information on who Shadow Serenity is and her connections to Selena.