Well,
Piper is only "unisex" in the sense that one could find a few examples in the 19th century in English speaking countries of men given almost every British surname as their first name. But it was never even moderately common as a boys' name so I think it's really pretty "feminine" in its entire usage pattern. It was created in 1950 as a female given name for the stage name of the actress
Piper Laurie, who was born
Rosetta Jacobs. I've never been able to like
Piper as a first name ever since I read an interview with Ms.
Laurie, who said she hated the name herself and was sorry she had let her agent convince her to use it. :)
I agree it would be best to pair it with a "feminine" middle name.
Piper Abigail,
Piper Arabella,
Piper Suzanne, -- something like that.