Things we know about the blackmailer:
-They somehow knew Ashe is a duplo (only medical records and Ashe's family had that info)
-They had eyes on Ashe and Vanessa during their make-out (inside Vanessa's walled backyard)
-The messages started in the early hours of day 10, after Ashe's dinner with Vanessa and Moira on the evening of day 8, after Ashe and Rachel had their make-up conversation and Ashe lied about not having romantic feelings for Rachel anymore on the afternoon of day 9, and after the sleepover party where Ashe lost her virginity with Zoey on Kate's bed late at night on day 9.
Also a relevant detail of the background lore in the game is the spyware that has been affecting several characters: The spyware has been a part of the game's background lore for a bit now, it was hinted at with the story of the leaked pics of the duplo streamer that overdosed in day 6, it is mentioned by Rachel in day 10 as a possibility of how the blackmailer could know about Ashe's sex (but Ashe reaffirms she never took any nude pics so her phone shouldn't have anything proving she is a duplo), and is once again brought up as the reason that caused a work emergency that stopped Fiona from hanging out at the beach in day 15 if not in the Kitten route.
With those things in mind, here are my opinions on the main potential suspects: