I figured out how to use Carty. Basically you need to build up her unique forte gauge by placing 3 swords on the field.
>hold attack for one sword >full combo for one sword >use E for one one sword >do a plunge attack to absorb 3 swords >the forte gauge will have swirling animations meaning you've absorbed the swords >transform into Fleur >build up a normal forte gauge >use burst >repeat