All except Please was published by Curve Circle. (dj)
Falls chronologically after Skies (dj). Hydra series is prequel to Lovers and Souls and Rules #01 (See Rules for Rules series Timeline)
Hydra has 3 volumes and several doujinshi.
Please chapter 5 - A Bird at Sea, A Door in the Sky
Hydra dj - Kiss
Hydra dj - Kiss After
Hydra chapter 0.5 - Skies (dj)
Hydra dj - Sono Mae no Hibi (The Days Before)