As others have pointed out, just figure out why the charisma is as low as it is and go with what fits best from there. Try and choose an aspect that will be recognizable but not too difficult to maintain throughout the game.
Agreed 100%.
Low Cha can mean a variety of things, like being:
- Unattractive, offensive or unclean
- Withdrawn and soft spoken, an introvert
- Brash & boorish
- So low key that one is easily ignored
- Prone to blurt out inappropriate statements...and not realizing why they're being hushed
- Perpetually depressed & morose
Essentially, "low Cha" could be anything that would make others ignore or avoid you in social situations.