Suggested fix (unconfirmed): Maintain Felix in his normal human form by selecting the top dialogue options, and then refuse to continue the battle with Onyx after the x-scene on day 14.
Suggested fix (unconfirmed): Reach the 15th day, complete over three additional SMS quests on your room's computer before that day, and choose the dialogue options 1, 2, and 1 to trust your allies and unlock Ending V.