Ashley was hardly a blank slate, with her being in a steady relationship at the start.
I think that
arisushimake meant to say that Ashley's personality is directed entirely by the player (for the game's purpose), she does not have her own, consistent, character or moral code in order for the player to chose it for them. From the first scene, the player can direct her to be the vanilla goody-two-shoes or an adventurous vixen, instead of slowly opening up to temptations, she can dive headfirst. Whether she has her own backstory, relationship problems or daddy issues is irrelevant because anyone can have those, they do not define a personality. If you remove the player's input from Ashley and try to get to know her as a person, you won't be able to, because she's very boring and incredibly inconsistent with herself.
Ian and Lena, on the other hand, aside from the player's input, have their own, fleshed out, personality that you can't really change all that much. It is made this way because Eva also needs to write their NPC counterparts. There will be several status quos for them, depending on your actions, but you won't be able to change their character all that much.