The terms “sempai” and “kohai” loosely translate to “senior” and “junior”. There’s a lot of meaning behind those loose translations.
Sempai is a term used to denote someone who is senior to you in experience. This can be in the dojo, in a company, or even a school context. If you’re in grade 9, you could say that someone in grade 10 is your sempai at school. A co-worker who’s been there longer than you is your sempai at work. Someone who’s trained in kendo longer than you is your sempai in the dojo.
Kohai is the flip-side of sempai. Someone who started kendo after you did is your kohai in the dojo.
Practically speaking, there are some things that go along with this relationship.
Sempai have “been there, done that”, so when it comes to asking questions about your kendo, it’s fair game to ask them. Keep in mind though that your sensei may have different perspectives than your sempai 😉
Further to that, sempai generally ask their kohai to do things around the dojo, like cleaning. While it’s everyone’s responsibility to keep the dojo clean, a sempai can ask a kohai to sweep the floor, but a kohai probably shouldn’t ask the same of their sempai. It’s kind of like asking your boss to do your own paperwork.
If you watch any Anime (Japanese animation), you’ll probably have seen the sempai/kohai relationship played out in myriad ways. Same for Japanese TV dramas. It’s a social convention in Japan, so it can be fraught with interpersonal drama.
So, does this change how you address people? That depends on your dojo. Here are my thoughts.
Sensei – should always be called sensei, or surname-sensei (i.e. Hayakawa Sensei). Also heard: given-name-Sensei or sensei-given-name (i.e. John Sensei or Sensei John). Not as common, and not as polite, IMHO.
Sempai – should only be addressed that way when they’re in a teaching role. So, if you’re not sure about a drill, putting up your hand and asking “Sempai, could you please clarify this drill?” would be appropriate.
Kohai – is not used to address people.
You can use sensei, sempai and kohai as a noun. “So and so is my (sensei/sempai/kohai).”
I hope this helps demystify the sempai/kohai relationship and how the terms are used. This is from my experience and perspective, so I welcome you to add your own.