Summer is the protagonist, or main character, of the novel. Because Rylant has written Missing May in the first person, Summer tells the story from her perspective as she experiences and understands it. Summer spent the first six years of her life in Ohio. After her mother died, she was passed around among her aunts and uncles. Summer knew that nobody wanted her, but that didn't dampen her spirit. She saved the love that she imagined was in her heart, given to her by her mother, for people who wanted to love her. When she met her Aunt May and Uncle Ob, things clicked, and it was love at first sight. She went home, to West Virginia, with her Aunt May and Uncle Ob to live with them in their broken down trailer. Summer was elated. May and Ob doted on her, treating her as though she was their very own child. She loved them and their eccentric ways as much as they loved her. She felt as though she had "died and gone to heaven."
Summer is 12 years old when Aunt May dies. She misses May terribly and is aware that Ob does, too. Summer recognizes that she and Ob are "not strong" anymore. She assumes the responsibility for trying to mend Ob's broken heart (an impossible task) in an attempt to hold onto as much of the old, familiar, comfortable life that she had known with May and Ob.
Cletus Underwood befriends Ob, and eventually, Summer. At first, Summer resents Cletus' presence. She is jealous of Cletus because he puts a sparkle in Ob's eyes when she can't. Over time, she begins to appreciate Cletus and is grateful for his attention to Ob. Because Ob spends more and more time talking to Cletus, Summer feels as though she has been left to fend for herself and she doesn't know what to do about it.
Things seem to be spiraling out of Summer's control. Ob talks about seeing May and neglects to get up one morning. Summer is afraid that if Ob falls apart, she will lose him and will then lose everything that she loves. She doesn't "know how to keep him tied to her." She feels as though they are both in a fog. She thinks the idea of going to Putnam County to see Reverend Young, a medium who talks to the dead, is crazy but she will do anything if it gives Ob the will to keep living.
Before going to Putnam County, Summer and Ob go to Cletus' house to get his parents permission to go with them. Summer realizes that Cletus has been ashamed of her because she has been reluctant to have anything to do with him (she thinks he is a little bit strange). She is embarrassed by her action. Since he has been spending time at their trailer, she has grown to respect Cletus and can now see the good in him.
The trip to Putnam County is peaceful for Summer. She is free of the responsibility of caring for everyone and everything — a burden that she placed on herself at the time of May's death. When they can't see Reverend Young (who has died) and talk to May, Ob is disappointed, and Summer is devastated. She thinks that Ob will give up on life because he can not see Reverend Young. She ". . . swallow[s] back the lump in [her] throat and pray[s] for something to save Ob and [her]."
When they get home, an owl flies over Summer's head and she remembers May. Over the past two seasons, she has been so concerned about Ob that she hasn't thought about May and hasn't been able to grieve for May. Finally, Summer realizes that May is gone and that she will never see May again. She cries her heart out for May. She accepts May's death and is able to let go of her fears. For the first time in two seasons, she feels free and certain that neither she nor Ob is going to give up on the life they have together.
Ob is Summer's uncle, who was married to May. He looks like a scarecrow — tall and thin. Because May always made him feel special, Ob never felt bad about being a "disabled Navy man" (he has arthritis). Ob is an artist. He makes unusual whirligigs — he and May call them The Mysteries. They are his interpretation of abstract ideas or things, such as heaven, love, dreams, fire, or death. Summer describes Ob as "a deep thinker . . . [who] was often getting revelations." May's acceptance and appreciation of Ob's eccentricities gave him the confidence to be himself and to be a "free thinker."
Ob deals with May's death by sitting in his old Chevy all night. He is devastated. He loved May as much as a person can love another, and he doesn't know how to live without her. One day shortly after her death, Ob has a revelation that May is with them, that her spirit is visiting Summer and Ob. He becomes frustrated and sad when he doesn't feel her presence again.
Cletus Underwood, a classmate of Summer's who is quite unique (Summer thinks he is a lunatic), begins to visit Ob and Summer frequently. Ob likes Cletus and seems to be fascinated by him. Cletus' visits and stories distract Ob from his grief and Cletus listens to Ob without judging him or making comments. Unknowingly, Cletus is allowing Ob to grieve for May in his own way.
Ob becomes quite depressed (a normal part of the grieving process). He doesn't get out of bed one morning, and when he does get up, he stays in his pajamas all day, crying in front of Summer — all behaviors that are unusual for Ob.
When Cletus arrives at the trailer with a newspaper clipping about a minister in Putnam County who communicates with the dead, Ob thinks they should go find this minister so he can talk to May. He feels hopeful and finally has a reason to live. Because Cletus is included in the plan to go to Putnam County, Ob and Summer go to visit Cletus' parents to get permission for Cletus to accompany them. Ob doesn't tell the Underwoods the truth about why they are going to Putnam County — instead of telling them about Reverend Young, he says the reason they are going is to cheer up Summer. Ob understands people and knows that "not everybody is as free minded" as he and Summer.
On their journey to find Reverend Young, Ob is quiet. He is sad and fearful about what Reverend Young might tell him. When they find out the minister has died and they can't speak to May, they get back in the car. Ob is disappointed and wants to get back home. They get in the car and drive towards Fayette County — past the capitol. And then, "Right out of the blue, he want[s] to live again." Ob turns the car around and goes back to the capitol. Ob can't bear the thought of disappointing Summer and Cletus.
Ob has finally accepted May's death. Ob, Summer, and Cletus, in a ritual to say goodbye to May, put all of Ob's whirligigs in May's garden. May's spirit is finally free, and Ob is free to go on with his life.
May was Summer's aunt and Ob's husband. Rylant describes her as a "big woman" (she had to "hoist" herself out of the car) who loved to garden. When she died, she was in her garden.
May was a homebody. She enjoyed being home and feared that if she left their trailer, it wouldn't be there when she returned. She suffered significant losses during her lifetime. Her family was victim of a flood when she was nine years old. Everything was washed away, and her mother and father died. May had a strong belief in "spirits from the next world" and always felt as though her parents watched over her — until she met Ob. Then they could relax and fly "off to that big church picnic in the sky."
May was reliable and dependable. She took care of Ob and Summer and was happy doing it. She provided a positive environment in which Summer flourished. May was an optimistic person, always looking for the best in people, and, in return, people showed her their best side. May allowed people the freedom to live life to the fullest and to be who they were meant to be. She was good for Ob and Summer because she always focused on their strengths, which increased their self-esteem and made them feel as though they could do whatever they set their mind to. May would have been happy to know that Ob's special whirligigs were in her garden and that Ob and Summer were now holding onto each other.
Cletus is one of Summer's classmates. He has long, straight, black hair that looks greasy (Summer doesn't think he bathes regularly), he has acne on his face, and he wears a funny looking hat that has ear flaps. Cletus is different from other people his age; Summer thinks he is a lunatic. He is a collector of unlikely items such as potato chip bags, buttons, spoons, plants, wrapping paper, and now, pictures that he carries with him in an old suitcase. Cletus uses his imagination to create stories for every picture.
Cletus first meets Ob and becomes acquainted with Summer when Ob notices him snooping around their old Chevy. As Summer and Ob grieve, Cletus becomes a "guide" for them in their search for May. Cletus and Ob become fast friends in spite of Summer's objections. Cletus seems to understand how devastating it has been for Ob to lose May. He begins visiting Ob and Summer often. Cletus' presence seems to have a positive impact on Ob, and Summer observes that Cletus has "some gifts." Cletus seems to live full of hope and confidence. He listens to Ob talk and is not judgmental. For example, when Ob stays in his pajamas all day, Cletus never comments; he simply ignores the fact that Ob has not gotten dressed all day. Cletus is insightful and perceptive. He tells Summer to "drop some of them bricks you keep hauling with you. Life just ain't that heavy." Cletus understands that Summer has taken on the responsibility of trying to fill May's shoes — a task that is impossible for her to accomplish. Cletus believes Summer is fighting herself and is not allowing herself to think freely or to be the writer he believes she is.
Cletus becomes Ob's "afterlife antenna." Because he had a near-death experience as a small child, Ob believes that Cletus' energy will attract May's spirit to visit again. Cletus shows Ob an advertisement for a medium — a spiritualist who speaks to the dead — named Reverend Young. Ob, Summer, and Cletus decide to take a trip to find her and possibly speak to May through her. Cletus is excited about the prospect of seeing the state capitol. He hasn't traveled much and, because he wants to be a "Renaissance Man," he sees the trip as an opportunity to move towards his goal.
After finding out that Reverend Young is no longer alive and after visiting the capitol, with which he is enthralled, Cletus goes home with Ob and Summer. He spends the night with them at their trailer, and then accompanies Ob and Summer to May's garden to help erect Ob's whirligigs. Cletus blesses the whirligigs by reading from Reverend Young's brochure. Unknowingly, Cletus has been a consoling presence for Ob and Summer during their grieving process.