give_trait is correct AFAIK, the problem is likely the name of the trait: you don't use the name of the trait itself but rather the "line" of traits it is a part of (GoodCommander, for example) You can find them all in export_descr_character_traits.txt in the main Data folder. If you then search within that file for the name of the particular trait you'll find the name of the family, and you follow that with a number for the "level" you want to give. Level 0 effectively removes the trait. The line for Flexible is called Corrupt, so the command should be:
Code:
give_trait Coelwine Corrupt 0
Antagonist