When an established character becomes largely different, exhibiting behavior contrary to what has been previously shown. This is not a matter of organic growth. Rather than gradually changing in response to events and experiences, a derailed character will exhibit shockingly unusual behavior that implies malfeasance or incompetence on the part of the writers. Fans motivated by Die for Our Ship will use Character Derailment in their fanfics to remove the character from the Official Couple which is in the way of their OTP (or just because they dislike them).