Considering what a pertinent character Annie Sullivan is to this play and to Helen Keller's life, it's interesting that we aren't introduced to her for a while. When she is finally introduced, however, we are given a detailed description in the stage directions:
The chair contains a girl of 20, Annie Sullivan, with a face which in repose is grave and rather obstinate, and when active is impudent, combative, twinkling with all the life that is lacking in Helen's, and handsome; there is a crude vitality to her.
This description leaves no question as to Annie's character. Even her features exhibit obstinacy! Ha! Even more is revealed later in that same scene. A last bit of information comes from Annie's mentor, Anagnos:
Always an Irish battle. For independence. ... You do lack some--by some I mean all--what, tact or talent to blend. To others. And what has saved you on more than one occasion here at Perkins is that there was nowhere to expel you to. Your eyes hurt?
Annie: My ears, Mr. Anagnos.
(And now she has opened her eyes; they are inflamed, vague, slightly crossed, clouded by the granular growth of trachoma, and she often keeps them closed to shut out the pain of light.)
A "handsome" Irish girl of only 20, beautiful except for her eyes that bear the obvious redness and puffiness of trachoma. When she's resting, her face looks "obstinate." When she's active, her face looks "impudent." Direct reflections of her personality. She has an inability to "blend in" with others, ... as well as a crass sense of humor. Her ears hurt (from Anagnos' comments), not her eyes (from the trachoma).
As you can see, the physical description revealed by Gibson gives the perfect introduction to the character of Annie and, in fact, reflects her exact personality.
Annie Sullivan is Helen's teacher, or governess. She is 20 years old and she got trachoma when she was younger, and that nearly left her blind. She had 9 operations on her eyes.
She is really strict to Helen because she knows what it feels like to be blind and she really wants to help her.