I really hate the crease under my butt. It is completely devoid of fat. What can I do help its appearance? (photo)

an in person consultation  would be extremely beneficial for you and the surgeon. Its impossible to tell based on the photo you submitted.  The skin at the bottom of the buttock can be trimmed to give your buttock a tighter appearance.   Please be sure to choose a Board Certified Plastic Surgeon who specializes in this procedure and is well known for it. 

Unfortunately this is a common problem after liposuction of this area. Although the result may not be perfect, there is usually some improvement with strategic fat grafting back into this area. If the fat grafting fails to produce a satisfactory outcome then a lower buttock tuck maybe your final option. Glad to be of help.

Hello dear!
Thanks for the question and provided information as well.
Based on your pictures, you are a good candidate for liposuction with fat transfer, to improve your buttocks, and I think you'll get very good results. 
In general, you need to be physically healthy and at a stable weight, have realistic expectations, be a non-smoker and your BMI needs to be 30 or less.
l recommend you to make an appointment with a board certified plastic surgeon to talk about your goals and anything you need to know.
Good luck :)