Well, her red eyes immediately give her tension.
The left elbow is in a passive mode but the right is higher and curved implying curiosity or severity. The lights reflets in pupils are crazy intense and are forming a triangle with the reflect on the mouth which make her even sharp. But this energy is counterbalanced by the blush, the spaces in the mouth, the dull haircut and especially the left horizontal eyelash.I like the line below the left eye to make age her and implying tiredness but with that fire in the eyes it's hard to believe. The range of her mouth interpretation to is very hard to contain, it honestly make me think about the Mona Lisa's mouth which, looking closely, you can't decide if she's smiling or if she's angry.
Like I said I really like this drawing for all the questions it raise, wish you the best for your future art pieces!
RE: Flat Portrait - Shiki Akane