It depends what you mean by personality
If you mean different behaviors to certain situations: use character scripts. You can use the default ones Larian provides and put them into a specific character. Select the character - go to the side bar (ctrl-B) - about 70% of the way down is Scripts - click the [...] that appears to the right o the box - a window will appear with a list of scripts - simply click on an Available script and then the right arrow to add it to the Assigned scripts. Now that character will have a new behavior.
As to what each of the default scripts do, some are named well. Others, you would have to check the script editor. All the templates exist in ...\Divinity - Original Sin\Data\Editor\Templates\Scripts\ for your inspection
If you want them to say different things use the Keyword editor to create such a dialog. Then (for non-global characters) put the dialogs file name into DefaultDialog (again in the sidebar, 40% of the way down). For global characters, assuming you are working with the main game scripts, use DB_Dialogs. Refer to Larian provided documentation http://www.divinityoriginalsin.com/download.php
(Useful Osiris Systems).
Some other potentially useful main game story scripts stuff is in my post http://www.larian.com/forums/ubbthreads.php?ubb=showflat&Number=556205#Post556476
see NPC stats section
For more advanced behaviors and personalities, you will need to do some of your own code writing with either the Story Editor or Script Editor.