What do we learn about Michael and Mina in chapters 7-9 of Skellig?

Michael, the protagonist and narrator of the story, meets his new neighbor Mina at the beginning of chapter 7. Her physical qualities are described in chapter 9. She is "little" with hair "as black as coal." Her eyes are deep; Michael comments that they are the type that "can see right through you."

Mina is direct and straightforward. She is the one who approaches Michael first, introducing herself and asking for Michael's name when his only reply is a silent stare. She also enjoys nature and art; in chapter 9, she chides Michael for scaring away a blackbird she had been trying to draw. She shares her drawings with Michael and points out the various colors in each bird's plumage. Mina is also rather fearless; she easily scampers up and down a tree and doesn't hesitate to point out Michael's "typical" misrepresentation of the blackbird's true colors.

By contrast, Michael is rather reserved. When Mina introduces herself, he is left speechless by her approach. He is also intuitive, understanding that the strange man outside is trying to order Chinese meals by using cryptic references to "27 and 53." Michael demonstrates great compassion for the stranger, even offering to allow the man to come inside the family's home. He worries about the man's well-being, asking numerous times if he can assist in some way. In chapter 7, Michael's investigation has left dust "clogging his nostrils and throat," and he is covered in dust and cobwebs as he backs out of the garage. We can infer that Michael and Mina are approximately the same ages from these chapters, but little other information is given regarding Michael's physical appearance.

How do Michael and Mina's perceptions of Skellig change throughout Skellig?

In David Almond's book, Skellig, Michael and Mina learn to see Skellig in a different way as time goes by: it's hard to say if they change because of him, or things around them make them change towards him, or both.

When Michael first sees Skellig, he is curious, but he also wants to help. As time goes on, he finds more ways to help Skellig, not just from bringing him Chinese food leftovers, aspirin, brown ale, and cod liver oil, but also when he asks the doctor at the hospital how he can help his "friend" who has arthritis. Michael easily becomes attached to Skellig—taking a risk by telling Mina about him, and moving him to a place where he will be safer.

Mina is an unusual girl: for her the world is full of wonder. She is naturally curious, but not one to dismiss another because of looks. While she may initially be intimidated by Skellig, she continually notes how extraordinary he is. Like Michael, she decides that she wants to help and protect Skellig, and the two sneak out at night to take this unusual person food and blankets. They develop a special kinship with Skellig, and under their care and ministrations, he begins to grow stronger.

[Mina] took off her cardigan. She folded it and laid it beneath his head.
"We'll bring you more things to make you comfortable," she said.
"We'll make you well. Is there anything you would like?"
I smiled.
"27 and 53," I said.
"27 and 53," he whimpered.
....
We smiled at each other. We looked at him, lying beside us.
"We won't be long," I said.
Mina kissed his pale cracked cheek. She stretched her arms once more around his back. Here eyes burned with astonishment and joy.

By the end of the story, the three have developed a deep and abiding love for each other. The children help Skellig to survive, they find a special friendship with each other that will continue after Skellig leaves and they have a new sense of feeling connected to the world.

Whatever gifts the children give Skellig, he returns  their love, and then visits the hospital, "flying" with Michael's baby sister, Joy— giving her strength to survive an operation the next day...even while he insists that she has helped him.

Michael and Mina discover that the value of a person is not in how he or she may look, but in who he or she is within. Everyone is deserving of kindness, and they learn that their kindnesses to Skellig are returned with his love for them. They did not help him to be rewarded, but find friendship and love have their own rewards.