It is the job of Sempai to guide and help kohai. It is the job of Kohai to always listen and do as Sempai says and does.
Every dojo is different, especially when you take into account all the different styles of training, let alone the different teachers and the communities they have built. The first Sempai/Kohai relationship however, is always the same: that of a sensei and their first student.
It is easy, as instructors/sensei/etc... to place ourselves on a "pedestal", whether on our own or by our students, but it is important to recognize that having that status of "instructor/sensei/etc..." is greatly determined by your having students. To be an instructor without students is the same as being a painter without brush or canvas; you may have the skill, but even that will dwindle without daily practice.
That is why it is always surprising to find instructors at any age who do not train regularly on some level, but who teach regularly. This may be a controversial statement, but I don't think so. Oftentimes these instructors will be overweight, inflexible, and dependent on students who cooperate with technique rather than actually ensuring that it is done properly.
In fact, the greatest teachers I have trained with, not only begin a class, demonstrate, and observe to help their students; they also join the fray and practice with them. This shows your students that you are not just invested in your ego. To be listened to and called "sensei" is a nice thing, but to do so without the primary drive of training honestly, would be a "false sensei".
Maintaining a sempai/kohai relationship, especially at the sensei level, can be key to maintaining a long term practice. This means not only having kohai that you are practicing with in order to drive them to improve, but also to be a kohai yourself. Identifying a sempai or sensei that you follow and listen to (note that I did not say "emulate" because to simply become a copy of your sempai or sensei would be to do them a disservice).
As a newer dojo, we are blessed with many younger students, both in age and rank who are already learning to develop these relationships naturally, helping each other develop their skill before, during, and after class.
The Sempai/Kohai relationship in martial arts is a sacred duty to both listen and learn.
See you on the mat,