Pink is a traditionally female color, so you could say that the issue of gender is first introduced in this poem with a pink wall. Especially in the 60s, when this poem was written, it would be unlikely for a boy's bedroom or bathroom to be painted pink. So, from this line in the poem, we can guess that the mirror has seen mostly female faces, and that the poem is concerned with women and femininity.
A woman bends over me (10)
This line introduces us to the only human character in this poem. But it doesn't make her very exciting – she's just a woman, bent over a lake to look into its waters and at her own reflection. We don't know what she looks like, what she's wearing, how old she is, or where she comes from. But this lack of detail serves a purpose – instead of being a specific woman, we can identify this woman with all women. This general, female figure takes the poem beyond being a story about one woman to say something about women in general.
She rewards me with tears and an agitation of hands (14)
Here, we see the woman showing distress. It's in this line that we see this woman's desperation to find what she really is – or perhaps her discontent with what she's finding in her reflection. This line shows that the importance of appearances to women can be destructive. To this woman, her reflection is important – important enough that she returns the lake again and again.
In me she has drowned a young girl (17)
This quote reveals part of why the woman is so disturbed by her reflection – she has seen her own youth disappear. Her loss of youth takes on a connotation of devastation by the use of the word "drowned."
In me an old woman Rises toward her day after day, like a terrible fish. (17-18)
This quote reveals the second half of why the woman is disturbed by her reflection. Now that the young girl is gone, the image of the old woman she is to become rises up at her, rushing at her as if it is going to swallow her into the waters. The last phrase in this quote, "like a terrible fish," shows the horror and disgust that this woman finds in the coming of old age.