Nancy and Holmes

A book by HexDSL (435 pages, full novel.) 

When Nancy first meets the detective who lives in her head, she assumed he was a symptom of her drinking, trauma of her parents’ death or just a homeless ghost. Then the first corpse turns up on the beach outside of her flat. That, coupled with the monster hunters, news reports and the sea monster stalking the town, makes her think it may be time to listen to her brain lodger. 

also on amazon https://www.amazon.co.uk/dp/B0CKNSYVVQ (but here is better!)