There are really only two characters, so to speak, in this poem: the speaker, widely believed to be Cummings himself, and the speaker's father, who has died (which we can determine because the speaker uses the past tense to describe his father and the man's life). It is not always (or even often) the case to assume that the speaker of the poem is the poet themselves; but, in this poem, most scholars agree that Cummings wrote the poem about his own father, Edward Cummings, who died suddenly when his car was hit by a train.
The speaker (or Cummings himself) clearly admired and loved his father; this seems an understatement, given the kinds of claims the speaker makes about his father in the poem. Rather than discuss the subjects typical of a eulogy—his father's occupation, his hobbies, his pursuits and interests, and so forth—he talks about how his father conducted himself during his life.
The overwhelming sense we get of his father is that he was tremendously loving and compassionate (no one would cry "vainly" in his presence) and that he was admired by a great many people because of his generosity of spirit and emotionally sensitive nature (he could "feel the mountains grow"). The speaker seems inspired by his father's legacy of love and has been moved to narrate this tribute as a result.
The Speaker's Father
The speaker's deceased father is eulogized in the most moving way—described as a person for whom "joy was his song and joy so pure / a heart of star by him could steer." This man seems to have been filled with love and joy and peace: so much so that any other person could depend on him to always possess these qualities. These lines call up an image of a ship using the stars to steer, and yet a star could rely on this man because he was so steady and dependable.