Charisma is gained randomly from the following actions/events

1. Complete school courses
2. Riding Murfin Madness
3. Decapitating Fairy
4. Puchalla Inn quest
5. Completing Level 13 of any of the 3 Simerian Temples (gives you 5 Charisma Stats)

Links
http://www.marapets.com/school.php
http://www.marapets.com/guillotine.php
http://www.marapets.com/rollercoaster.php
http://www.marapets.com/inn.php