Here is the full list of all 13 Dangerous Relationship achievements.
Completed All Main Scenarios, Sequels, and Date Scenarios.
Completed with "Akira" Good Ending.
Completed with All Endings.
Completed with "Taiga" Good Ending.
Completed with "Subaru" Good Ending.
Completed with "Kei" Good Ending.
Completed with "Ryouhei" Good Ending.
Completed with "Subaru" Best Ending.
Completed with "Kei" Best Ending.
Completed with "Akira" Best Ending.
Completed with "Taiga" Best Ending.
Completed with "Ryouhei" Best Ending.
Completed the Prologue.