Right then - now that I'm home, and can type at length...
Sakura (Heroine) - In many ways, she sees the world as her sandbox. Given enough willpower, time, effort, and luck, she can mold at least part of it to her will. One trait she has (that's common among pretty much her whole family) is a drive for perfection, with the realization that while it's impossible, it's a worthy struggle as it hones her skills and abilities to a high-level. Thus the hours upon hours of slaving over ship fits, tactics, physical exertion, and so on. Her pride and strenuous efforts to be honorable and keep her word are both disadvantages in this cutthroat world, as is her personal quest for glory and fame.
Akusa (Sage) - One could call her "the intellectual". She's quiet, thinks deeply about things, tends to keep her mouth shut when she's not sure about something, and beat herself up severely when she fails. Perfectionist at heart, like many other Nihil family members, she is reluctant to try things when she's not sure what she's doing, and is in many ways inclined to study a problem to death instead of leaping in headfirst and going for it.
Ereka (Heroine) - Probably the most ideologically driven of the bunch, she is passionate about her cause (in many ways, she's the primary driver behind the corp, not Sak, a radical change compared to past history). She takes training and developing her skills seriously, but is also under much more pressure than Sakura is, given her personal combat inexperience. In addition to trying to prove that her idea for a corp can exist, she's also trying to prove she's up to the task... that stress in many ways is only mellowed by her interactions with other characters, and a (probably excessive) enjoyment of alcohol. A socialite at heart, really.
Eri (Ruler) - She is a tough cookie to figure out. The best way I can picture Eri is as a take-no-prisoners lawyer, a shark who shows no mercy to her opponents, and views herself as better than most others she interacts with. Oftentimes, she comes off as a bitch, and even to those she's more comfortable with, she always has this edge to her. In many ways, she's insecure, and can be loud and authoritarian to anyone she deems "beneath her". As her player... uh, I hope she lightens up over the course of time like Ami did? We'll see if that happens...
Ami (Explorer) - She's probably the least gung-ho in the family about capsuleerdom, and in general she's always been alone. Whether as the last child out of the nest, being the baby of the family she is, or the youngest in the pod and generally left to her own devices, she's had to keep herself entertained. If this thread would have come up a year or two ago, I'd say she'd have been a rebel, but as more time's passed she's mellowed out, and has begun to appreciate and explore the cluster around her. She's still in many ways solitary, but she's at least amicable these days.