Behind the scenes has the answer.If i had no additional information I would have easily guessed Layla was the creator's intended personification - she has many more scenes with unique quirks - like when they're stretching at track event and she looks like a dork when she goes up for a kiss when he's not there....I've never seen something like that in an 'adult game' before and it was great, like how actual people act.
Layla and Erynn are the "pre-planned" stories, while most other girls are "change of plans", "expanded roles", "last minute additions".
The game feels weird when you are not on the Layla path, so much time dedicated to a girl you aren't chasing.