Hexadecimal is a level 2 virus. Chaos virus to be exact. To quote Bob "Hex is one loopy lady." Hexy is insane and loves chaos. It is quite obvious that Hex has an odd fascination with the blue guardian. She goes even more insane after her brother, Megabyte, captures here and makes her into a weapon using her energy.
Hex's face is a mask. To change expressions she has to change masks.