Neutral protein filler is a hair product that can be used to add volume, texture, and body to your hair. It is often used in conjunction with other styling products to create a variety of different looks. Neutral protein filler is made from keratin, which is a type of protein that is naturally found in your hair. This product can help to temporarily fill in any gaps or spaces in your hair that may be caused by damage or breakage. Neutral protein filler can also help to make your hair appear thicker and fuller.