There are two main characters in Sylvia Plath's "Morning Song." Arguably there are also some implied characters, chief among these being the husband of the speaker and possibly a family or group of friends beyond this.

The narrator of the poem is a new mother. She is not identified, but given the nature of most of Plath's poetry, we can assume that this is probably Plath herself, with the poem representing a means of exploring her feelings about having become a mother. The narrator finds motherhood difficult, or at least confusing. She does not seem to have adapted quickly to the idea of having a child, although she knows that the baby was born out of "love" between herself and her husband.

At the beginning of the poem, it is a midwife who slaps the baby, provoking a cry which seems to "set [it] going" like a watch, a mechanical object. This active interaction between midwife and baby is more than there is in the poem between the mother and her child: later, she stands "blankly" while the baby is displayed to relatives, and she listens to the child breathing at night—which registers more of an interest in its wellbeing or at least continued existence—but she does not describe touching the baby or, really, interacting with it.

Clearly, the mother in this poem finds it difficult to look at her baby without seeing it as something which diminishes who she is. She is now drawn as "a mother," but she feels she is no more the mother of a baby than a cloud is the mother of the wind which will soon blow it away. The image of a cloud watching in a mirror as this happens seems to illustrate the mother's fears.

The other key character in the poem is, of course, the baby itself. The gender of the baby is not identified; it is addressed in the second person as "you" throughout the poem, as if the mother is trying to approach it and understand. The baby is compared to manmade things such as a watch and a statue to be shown off proudly. Later, it makes a tentative step towards being a living creature when the speaker compares it to a "cat," but it still does not seem to feel, to its mother, like a real person. The baby is too young to interact much with the world around it, confined to "moth breath" and trying to make its first tentative sounds known.