anorexia

Anorexia or Anorexia Nervosa, is an eating disorder characterized by low weight, food restriction, body image problems, a fear of gaining weight, and an overpowering desire to be thin — often while the sufferer is already underweight.

This tag can apply to characters expressing a desire to be thin or appear extremely concerned about their weight, while already being thin, usually to an unhealthy degree (appearing emaciated or at the very least, sickly and very skinny).

For tags describing appearance see:
skinny — characters with narrow, thin figures, usually to a more healthy/natural degree
emaciated — for characters that are practically (or sometimes literally) skin and bones