As for the characters and fluff....AToW makes this easy with it's character generation method.
Simply think of the character being a baby at the start, then at each stage come up with a little story as to why the choices you make at each stage happen:
For instance:
Character Name: Kane
Stage 0: Federated Suns baby from the outback region of Crucis March, Son of a farmer farther and a school teacher mum - Chose to learn how to draw (Arts: Drawing) from a young age sitting on top of the rolling hills of the area where he lived with his parents looking out over the endless grain fields, with the view of the local mountain range in the distance.
Stage 1: Early childhood (up to 10 years old) Chose 'Farm'...spent most of his time out in the fields playing games, and occasionally helping dad with basic farm duty. Used the flexible XP to offset some of the illiterate trait, increase his arts skill and learn German and increase his English skills and Wealth trait. Chose to take Interest: SLDF History (all kids love the military) and Interest: Arts (to enhance his understanding of his skill with a pencil).
Stage 2: Late Childhood (end at age 16) Chose 'High School' due to mums profession and ability to get him erolled at her school (which a lot of other kids can't get do due to their environment on this backwater planet). At school Kane increased his Interest: Arts skill as his primary interest, then also increased his Interest: SLDF History, further increasing his personal interest in joining the AFFS and helping the Davions protect the interests of Federated Suns. He uses his flexible XP to remove the rest of the illiterate trait and the rest of the negative Wealth trait, but then also decides that he wants to earna bit of money and increases the wealth into a positive score. This Kane achieved by doing a paper round in his local town to earn him pocket money and lay the foundations of the knowledge of how to earn, save and spend money when he becomes an adult.
Stage 3: Higher Education. Having graduated from school with good grades Kane decides that he wants to push to be a MechWarrior (who doesn't after all!) so he uses some connections he has made through his school teachers to get into one of the lower end Military Academies...as it happens his head teacher is an ex MechWarrior that happened to train at the New Avalon Military Academy (now the NAIS) and gets Kane a placement with the elite school. (Kane has to pay the prerequisites to get into the academy, but the player thinks it's well worth it!
Kane goes through basic training, MechWarrior training and then decides he wants to spend a second year perfecting his Mech skills. At this stage the flexible XP are going into improving his mech skills and attributes, etc.
Stage 3: part 2: Officer Candidate School. Kane has proved himself a capable MechWarrior and it is suggested that he attend officer training and so he attends that school, from which he exits as a newly promoted Leftenant (his parents are well pleased with how their son is prgressing, even if they are worried that he might get killed, something that causes friction between Kane and his farther.
Stage 4: Tour of Duty. Once promoted from officer school Kane is posted personally by the Prince to take command of a lance of scout mechs on the Fed Suns / Blake Protectorate boarder. Here Kane proves his worth as a staff officer, earns a promotion to Captain and ends with a medal for services to the Crown.
We start the campaign with Kane in command of his light mech battalion on the boarders of the Blake territory.
And there you go - I threw this together as I was typing it and didn't think too much about it as I went. It's simple when you go through in these stages :)
