Use the -s suffix. I don't know how plurals are formed in Japanese, but I do know that trying to use foreign plurals in words borrowed into English usually causes problems - the Internet is peppered with mistaken Latinate endings (for example the ubiquitous syllabi), and similar hypercorrect parades of ignorance are best avoided by using English plurals.The user thinks s/he's showing off his/her greater knowledge, but at best s/he's making things unnecessarily complex, and at worst s/he's making him/herself look ridiculous. Use -s - that's what Fowler, for example, recommends.
You could also avoid the problem by supplying a generic noun: 'ninja/samurai warriors'..?
PS If your teachers say 'syllabi' (as many of mine did) it may be politic to leave them be; or - if you're on good terms with them - you could (privately) observe that the plural is syllabus with a long U.
PPS I was getting 'syllabus' confused with 'prospectus'. I am not correcting this here (and thus making the later discussion of my mistake confusing); see posts 4 and 5
PPS Abject apologies to OP and 5jj, who I unwittingly invited to share in my error!
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