Robbie is a very dynamic character who has to face extremely difficult situations throughout the course of the novel; those situations will inevitably change him. We learn quite a bit about Robbie as a young boy, growing up on the estate, through character descriptions in Cecilia's mind, and also though Robbie's own reflections. He was a happy boy, hard-working, and in school, very ambitious. When we meet him as the novel begins, he is filled with hope, life, dreams, and the sure confidence of having the world at his feet. He nurses dreams of being a doctor, and is fully confident that he can achieve that goal. He has the assurance, ambition and innocent dreams of a youth who has lived a good life.
After the events that change everything, we meet Robbie as a soldier, fighting in World War Two. Gone is his confience; instead, he is unsure and hesitant. Gone is his happy dreaming; instead, he is filled with impatience and cynicism. He is hardened, bitter about events from the past, and restless. He doesn't like fighting the war, and instead is anxious to get his life back in order from the time that he lost. Instead of an outspoken leader, he is a quiet, melancholy leader. There is a sense of desparacy about him, and even hopelessness. One can't help but understand the changes that he has gone through, and have sympathy for what he has had to live with.
I hope that those thoughts helped; good luck!