If you have seen the Gundam Wing anime, this offers you excellent insight into the lives of the characters. After reading this, you really understand why they do certain things in the show and it helps complete the picture. You learn about all the pilots and Relena and how they were before the start of the show.
Heero was raised to be an assassin by Odin Lowe, the man who assassinated Heero Yuy. He was the one who taught him the phrase, "follow your emotions" that he tells Trowa and others.
Relena was a quiet, depressed girl who met someone who looked very much like Heero years before the start of the show. He gave her a reason to hope and believe.
Duo was a war orphan and he and his orphan buddies would break into Alliance facilities to get food for themselves, hence his knack for breaking into places. He was taken in by Maxwell church where a nun braided his hair and a priest tried to teach him about God. Because he has only seen death, he cannot comprehend there is a God of life. So, he decides there is only Shinigami, the God of Death.
Trowa's story is interesting because it turns out he is actually related to Catherine. He was from a family of circus performers who were escaping in a wagon from a battle zone. Trowa is a toddler and falls off the wagon but his family cannot go back for him. This is why he doesn't know what his real name is and calls himself No-name. Trowa is raised by random militia people where he meets Middie Une (who may or may not be related to Lady Une). The two of the get along very well, but it turns out she is a spy. However, the feelings she had for him were real and she is the first person to really care about him. It then replays the segment of Trowa's past we learn about in Endless Waltz where he assumes the real Trowa Barton's name and place as a Gundam pilot.
Quatra is the youngest in his family and his 29 older sisters are all test tube children. However, unbeknown to Quatre, his mother died giving birth to him naturally. Therefore, he believes he is a test tube child, because that is what he is told, and he feels as if his life is worth nothing. The Maganac Corps is a group of soldiers who are all test tube children. They show Quatre that there is meaning to life and he comes to terms with being a 'test tube child'.
Wufei is not meant to become a Gundam pilot. He was actually a martial arts genius and a bookworm. His fiancee, Meilan, was meant to pilot Shenlong. She believes she is like the god, Nataku, and calls herself so. However, during a battle, she is injured and dies in Wufei's arms. He believes her spirit inhabits Shenlong, hence why he calls it Nataku, and he fights to honor her.