It takes a long time for Walcott to let us in on this, but Maud Plunkett is kind of a betty—a really white, Victorian betty:
All her county shone in her face when the power
was cut, and the wick in the lamp would leap, as live
as the russet glints of her proud hair (LXI.i.304)
We see her taken up by domestic tasks: growing tropical plants in her garden, embroidering a fantastic quilt full of birds and flowers, appeasing her husband by memorizing bad histories. Her other main action is to refrain from clawing out Helen's eyeballs.
Maud feels displaced on the island (unlike her husband) and longs for the greenness of her own island (a.k.a. Ireland). Despite her genuine affection for Dennis (check out the flirting in LI.iii.259), Maud cannot get behind his passion for St. Lucia or his desire to give the place a history. She genuinely thinks that Dennis is a bit wrong in the head from his war wounds.
But for all that, we don't know much about Maud. She is defined in relation to her husband and Helen, but beyond that she has very little in the way of unique identity. Perhaps this is because our poet isn't really isn't that into her—at one point, he says: "I knew little about Maud Plunkett" (LIII.ii.265)—or because he's keeping a "filial" distance from her.
We know, for instance, that Maud has a little bit of the narrator's mother in her. Walcott plays with perspective to shed some light on Maud's character, assuming Major Plunkett's point-of-view to give us some insight:
I had looked up from the green baize with the Major's
face from the ornate desk to see the light going
from her image, and that image was my mother's,
whose death would be real, real as our knowing. (LIII.ii.266)
There is something about Maud that links her intimately with Walcott's lost mother. This makes her a solitary character, a little sad—and ultimately doomed. We know that she will die (Plunkett believes that she is embroidering her shroud), and it is in her death that she becomes a much more dynamic image.
When Major Plunkett is confronted with losing Maud, we get a rich history of the life between them, including the love letters and the embarrassing near-love-making moment at the beginning of their relationship.
Ultimately, she's a good-natured woman living with a fond but unsympathetic husband. Her death also gives Walcott the opportunity to do something pretty nifty: He (as narrator) attends her funeral and talks to her husband. So hey, at the very least, Maud Plunkett affords our poet the opportunity to be interesting.