A dangerous serial killer charms his way into a single Mom’s life which brings a talented FBI agent out of retirement to work on the case whilst supernatural Neptune is also awakened to help.

It can have both.

It can have both.

The problem with this logline is that is starts off as a serial killer story and ends as a fantasy story.

Please don't post your logline twice the next time.

On to your logline:

It's a little bit confusing. First of all, you shouldn't use the names of the characters in a logline. Only use character traits or descriptions like "an abondened single mom" or "an alcoholic fbi agent". We don't need to know their specific names in a logline.

Also, what does the hero "Sea God Neptune" have to do with the fbi agent and the story in itself? I can't read any connection out of your concept.

Your different plot elements have to be intertwined in your story and all be part of a larger context. I don't see it here with your logline.