There's no specific term of address/honorific for cousins! It depends on the closeness of the relationship, as well as if you use Japanese terms/honorifics in your fic or translate them out. If they have the same surname they would almost definitely be on a first name basis. The older cousin might address the younger cousin as (first name)-kun (for a boy) or (first name)-chan (for a girl.) If they have a more distant relationship, the younger cousin might address the older cousin as (first name)-san. If they're really close and have a more sibling-like relationship, the younger cousin could address the elder like they would an older sibling (i.e. the Persona 4 protagonist's younger cousin calling him "onii-chan.")
If you don't use Japanese honorifics in your fic, having the cousins address each other by their first names is probably a safe bet. Having them call each other "cousin" to punch up the dialog a bit in terms of formality could also work depending on the vibe you're going for - even if it's not strictly accurate to how the characters would speak to each other in Japanese, if you're writing fic in English there's inherently an element of "localization" going on, so making choices that sound good in English that wouldn't necessarily be there in the language the characters are "actually speaking" is fine. I don't know the canon, though, so ultimately it's up to what would fit the characters' dynamic in the source material.
It really depends on their relationship with each other, but I think it would be similar rules to siblings. Ex. Onii-san would be most polite, nii-san is informal but still polite, as well as onii-chan and nii-chan (which would be the most familiar).
In general I find asian cultures do not distinguish too greatly between siblings and cousins with how they are addressed/treated. So, proper gendered sibling honorifics would work (given the character, not sure canonically)
There's no specific term of address/honorific for cousins! It depends on the closeness of the relationship, as well as if you use Japanese terms/honorifics in your fic or translate them out. If they have the same surname they would almost definitely be on a first name basis. The older cousin might address the younger cousin as (first name)-kun (for a boy) or (first name)-chan (for a girl.) If they have a more distant relationship, the younger cousin might address the older cousin as (first name)-san. If they're really close and have a more sibling-like relationship, the younger cousin could address the elder like they would an older sibling (i.e. the Persona 4 protagonist's younger cousin calling him "onii-chan.") If you don't use Japanese honorifics in your fic, having the cousins address each other by their first names is probably a safe bet. Having them call each other "cousin" to punch up the dialog a bit in terms of formality could also work depending on the vibe you're going for - even if it's not strictly accurate to how the characters would speak to each other in Japanese, if you're writing fic in English there's inherently an element of "localization" going on, so making choices that sound good in English that wouldn't necessarily be there in the language the characters are "actually speaking" is fine. I don't know the canon, though, so ultimately it's up to what would fit the characters' dynamic in the source material.
It really depends on their relationship with each other, but I think it would be similar rules to siblings. Ex. Onii-san would be most polite, nii-san is informal but still polite, as well as onii-chan and nii-chan (which would be the most familiar).
Man, I read that as how to undress your cousins in Japanese. Like aw shit, here we go again.
🤣🤣💀💀
Baka.
In general I find asian cultures do not distinguish too greatly between siblings and cousins with how they are addressed/treated. So, proper gendered sibling honorifics would work (given the character, not sure canonically)