The underside of my hair toward the back is straight most of the time too. Sometimes it'll do a little flip at the very bottom, but pretty much not even that. I think it helps that it's only in the back, because my hair usually ends up coming over my shoulder anyway. Also, I keep that section cut a bit shorter so it doesn't stick out under the rest of my hair. Whenever I've mentioned it to people, they usually say they hadn't noticed.
Fine Texture, Normal Porosity, Normal Elasticity. Dyed to brown a few months ago.
Lowpoo:
Trader Joe's TTT Shampoo
Rinse Out:
Yes to Carrots, AOHR, Aubrey Organics Rosemary Peppermint
LI: Yes to Carrots
Stylers: Shea Moisture Curl Enhancing Smoothie, LA Looks Sports, Suave Professionals Captivating Curls Whipped Mousse