For the character, my choice is Heather Hoodhood. The skill that I would give her (teach her) would be the skill of empathy for others. Heather is a fairly self centered character. She cares about herself more than anybody else, and she does not respect the opinions of other people either. She is especially loathsome to her parents. Granted, at times she does seem to care for Holling. For example, she helps him pull down the embarrassing posters. Other than that, though, she is solely focused on herself. She would benefit from trying to understand the feelings of other people, which is why I choose the skill of empathy.
If your question is asking about a physical skill, I choose Holling. I would give him the runner's skill of rescue breathing. Holling is a runner, and advanced breathing techniques would help him run better.