The Dentist Assistant is a young Hispanic woman, acting as a Nurse/Assistant to the Dentist.
During the events of 'The Dentist', she meets Dan (who she calls Danny) for the first time. Chris commented on her being cute, and Dan was apparently rude to her (which Chris reprimanded Dan for) She attempted to wrestle Dan into a dentist chair, but was unable to properly do so, until eventually resorting to grabbing a mask and dosing Dan with laughing gas.
She was last seen prior to Dan's surgery. Based on the dialogue, she was not present during Dan's childhood encounter with the dentist(likely having been a child herself at the time)