A friend who teaches Kendo in the UK’s West Midlands told me about another local martial art (not telling you which one), instructor who has the word sensei tattooed in large gothic letters on the back of his bald head. To my mind not a good move. As well as the potential problem of a permanent redundant label if he gives up, (a bit like tattooing a lovers name before you get chucked), it demonstrates a complete lack of understanding of what the word means.
Sensei is a courtesy title, so you can’t apply it to yourself without looking at best, confused. The literal meaning is “born before” and in normal Japanese society it is reserved for doctors, teachers, lawyers, politicians etc. If you are inside one of these groups, then you would use the sensei title for senior or more experienced colleagues, whilst using san or kun as appropriate for juniors. In Japanese, sensei is a suffix, so the correct usage is after the person’s name. It is Smith sensei, not sensei Smith. In kendo sensei is usually reserved for 6th dan upwards and even then infers that he or she is older or more experienced. Fighters in the Kyoto Taikai are announced as such and such sensei, whereas younger elite kendoka are announce as so and so senshu in the All Japan Championships.
Sempai too gets a great deal of misuse. In its natural setting it indicates someone older, your senior at school or perhaps someone who started kendo, or at the dojo before you. In the UK the term, is applied to the person at the head of the student line who shouts instructions. Within the original meaning of sempai and kohai (the junior member of the partnership), relationships are fixed on a lifelong basis, regardless of eventual status changes. I was unfailingly amused to regularly hear an 80 year old addressing a 79 year old as kun and kimi (honorific normally used for young boys and “you” form used for children). Of course both sides of the partnership have their own responsibility. The kohai normally packs and carries bogu bags and the sempai softens the blow of regular criticism by buying the drinks