Day One
Day Two
Day Three
Day Four
On the 4th day, choose "Try protecting".
In addition the affections points with Nami less than 3.
Also, if Nami's affection points are less than 2, you'll see a different bad ending on Day 3, so it seems the condition is that Nami's affection points are equal to 2.
Following the choices up top, the latter half of Day 1 draws closer to Yasumi in order to not let Nami's affection points get too high.