What does the red skull icon on adventurer portrait mean?
What does the red skull icon / stamp mean that is on the top left corner of the adventurer's portrait along the right side of this mean?
It appears to me to have no bearing on negative quirks, permanent or temporary. Nor does it seem to be associated with other aspects of the character diseases or positive quirks.
The red skull/stamp indicates that the adventurer is currently part of your expedition group.