Main ending: who becomes human?
Major ending spoilers. After the final battle, Davy devours KOM and Julia returns to human form. Davy remains a zombie. Their first date still happens, which is also reflected in Our First Date, After All.
The outcome is not “both characters become human.” If your goal is to reach this ending, follow the main-story walkthrough through the Brain Cores and final showdown.
What about a “true ending”?
Some postgame videos use “true ending” or “secret final boss” in their titles. That wording alone does not establish a separate ending in which Davy becomes human.
In the postgame dialogue, Davy still wants to find a way to become human, his friends make no guarantee and the group continues diving. Treat that as a continuation of the journey. A definitive all-ending trigger list is not established here.
Story completion versus Hollow
The first-date scene completes the main story. Everent Hollow belongs to postgame progression. For the trophy that asks you to enter Hollow, entry is the objective; clearing 100 floors is not an extra requirement for that trophy.