In "The Red Convertible," Susy represents natural innocence and freedom. She is like a guide (angelic perhaps with her abnormally long hair) as she takes Henry and Lyman to Alaska, which Lyman says is a place they "never wanted to leave." Susy's family is welcoming, feeding them and allowing them to live there. Their place in Alaska is like a dream world. Lyman says, "You never feel like you have to sleep hard or put away the world."
They get there in summer and because they are so close to the North Pole, the days are extremely long. Lyman also notes how quickly things grow there. This place in Alaska is full of light and life, a place where you don't even feel the need to sleep. Lyman says they would doze off and wake up like animals in nature. They were completely content and in tune with their surroundings. It is a spiritual but also natural description of this place: heavenly and Earth at its best.
Susy is the one who took them there. In doing so, she showed them an idyllic experience of nature and life. In a way, she was giving Lyman and Henry one last innocent, sublime experience before Henry goes off to war. For Henry, Alaska must have seemed like Heaven compared to the hell he would experience in the war.