As Far As The Eye

Traits are unlocked on the end game screen after completing a successful Quick Game scenario, wherein the condition to unlock the trait is met.

The trait Fragile is unlocked by having a pupil be impacted by the Boredom vagary. Vagaries are random events that occur during gameplay. One of the vagaries, Boredom, makes a pupil sick if they're not currently working (e.g. healing, moving, or not doing anything). The turn information shows when Vagaries will occur, and the upcoming vagary is typically announced a few turns before. Have a pupil get impacted by this, and heal them with a Potion or let the sickness run its course, and complete the run to unlock this as well as Delirious.