I might be wrong, but a mischievous personality can be obtained by having a high enough cunning stat. That may have been changed or taken out entirely though.
Exceeding 75 Corruption will change Personality from Champion to Dark Knight; going below 25 Corruption will change Personality from Dark Knight to Champion. As such, the PC is a Champion by default, since they start with 0 Corruption.
Changing Corruption between 25 and 75 won't change Personality, but writers can make variants (including PC dialogue) based solely on Corruption.
Having a bimbo perk will change Personality to Bimbo, overriding both Champion and Dark Knight unless and until that perk is removed.
The Mischievous and Bro Personalities from TiTS no longer exist, though the former can be replicated somewhat by having high Cunning PCs make witty remarks when appropriate.
I might be wrong, but a mischievous personality can be obtained by having a high enough cunning stat. That may have been changed or taken out entirely though.