Reyzon is not used to smiling, so she is doing her best to push her facial muscles to make a "happy expression," but she is almost unsuccessful and can barely muster a grin in the hopes that Aki and Kyoun would mistakenly believe she is joyful. Even at times, Reyzon and her friends would make fun of the fact that her cheerful face almost resembled an android's.
It's looks like Grit's a fan too