Alternate Ending Guide
Thanks to Sword_of_Dusk and YangustheLegendaryBandit
Standard Medea ending: Just play normally. This is the one you'll get. If you fulfilled the requirements for the standard Jessica ending, you'll be asked some questions. Answer "no", then "yes".
Standard Jessica ending: After events in Arcadia, speak to Rosalind in Alexandria to get Alistair's Armor. Then, sleep in Alexandria's inn. After beating the final boss, answer yes twice to the questions that will be asked at the end of the credits.
To get the other two endings, you must clear the post game to receive the Argon Ring.
True Medea ending: After clearing the final boss again, talk to King Clavius and say "yes" to show the Argon Ring you got in the post game to him. If the requirements were met, answer yes twice to the questions asked by King Clavius.
True Jessica ending: Again, beat the final boss talk to King Clavius and say "yes" to show him the Argon Ring. You'll be asked some questions by King Clavius, to which you should answer "no", "yes", and "yes".
You can get any of the four endings after you obtain the Argon Ring for the two post game endings.
To get either of the original two endings just leave the Lord High Priest's home without speaking to Clavius, and you can get the standard Medea ending or standard Jessica ending.