Re: Sempai kohai differences
In Japan, anyone senior to you is your sempai. Anyone junior to you is your kohai. As Japan is a tate-shakai -- a vertically-structured society -- this is always true, and it's immutable. You don't have to like your sempai, but if he joined before you did, he's your sempai and you're his kohai. Neither of you can not be.
This can be a bit disconcerting to western cultures, particularly in the U.S., I think. But for Japanese people, this is like a fish being in water. (Consider, for example, that Japanese has no word for simply "brother" or "sister". It is either "older brother" or "younger brother", "older sister" or "younger sister". The hierarchy is always explicit.) So in Japan, who's your sempai and who's your kohai is simply a matter of fact. You have sempai you are close to, and sempai you don't like, and kohai you especially look after, and kohai you're not particularly close to. Inherent in the relationship are certain societal obligations, but these largely extend to social ritual (who sits where, who pours whose drink, etc.) and can be as strong and meaningful (or not) as the relationship dictates.
What about a kohai who's older than you? A sempai younger than you? What about that kohai who passed you in rank? That sempai whom you have clearly surpassed in ability? Pshaw. Japanese people navigate these waters like in a motorboat in a pond on a calm day. Because the sempai/kohai relationship is always on, with everyone within the group, it's background noise. So these situations aren't complex at all. Granted, they may occasionally require finesse, but because all Japanese relationships are hierarchal, it's the kind of thing they deal with everyday.