In David Almond's Skellig the main characters in the story are all extremely isolated. Michael and his family have moved into a new house, but his new baby sister was born prematurely, and his mother spends a lot of time at the hospital; his dad tries to occupy Michael by doing things around the house, but the boy is feeling disconnected, especially in a "new" house that needs a great deal of work.
Michael makes friends with Mina who is also isolated, though in many ways it suits her personality. She is home-schooled by her mother and her lessons are really interesting—much different than what is taught in public school. She knows a great deal about birds, for example. (This is a very important piece of the story, as is the house her family owns with owls in the upper rooms.)
Sometimes Michael struggles to make his friendship work with Mina, especially when his friends at school make fun of her, and of Michael for hanging out with her. However, the thing that draws them especially close is Michael's discovery of Skellig, and Mina's desire to help Michael save this "supernatural" creature.
Skellig is the most alienated of all: he has no family, he cannot go out into society, he lives in Michael's old garage in a heap of trash, and he has been eating discarded food and bugs. When Michael finds him, it seems that Skellig will not live much longer.
So a major theme in the story is alienation. The second more important theme is friendship. I see a third strong theme as stepping out of your own "comfort zone" to help someone desperately in need. Michael and Mina see something unique and special in Skellig where most people might be afraid. When the two friends decide to save Skellig, it is an innocent gesture—from the heart: all they want to do is help this fragile creature (who first refers to himself as "Arthur Itis"). In the true spirit of giving, Skellig, whose condition does improve, repays Michael for his original kindness: for helping him and never giving up—Skellig saves Michael's baby sister's life, though Skellig says that it was she who saved him:
"You went to my sister," I said.
"Hm! Pretty little thing."
"You made her strong."
"That one's glittering with life. Heart like fire. It was her that gave the strength to me."
In all, the story is about sadness, fear, desperation and loneliness, which is countered by friendship, hope, tolerance and selflessness. Michael's family is desperately afraid of losing baby Joy. Mina is bright but cut off from the world of people—Michael's friendship allows her to belong and give back to a cause that makes their relationship stronger. Michael meets an amazing creature at a time when he is terribly distraught—helping Skelling helps him—Joy is not the only one in the family with heart. Skellig, too, demonstrates the power of kindness and love. So the story is also about miracles.
If I had to write an essay, I might write:
Love and kindness, shown to others, has the power to change the world.
However, Michael asks Skellig...
"What are you?"...
"Something like you, something like a beast, something like a bird, something like an angel." He laughed. "Something like that."
In Hebrews 13:2, the Bible says:
Do not neglect to show hospitality to strangers, for thereby some have entertained angels unawares. (ESV)
So I might write instead:
Be careful not to judge a book by its cover for "some have entertained angels unaware."
I like the second one particularly.