Why does Trafalgar D. Law use 'Ya-や(屋)' suffix while addressing others? Using ~-san, ~-kun, ~-sama while addressing other people is common in Japanese culture.But Law uses ya as suffix while addressing people, like in the nicknames Strawhat-ya for Luffy and Blackleg-ya for Sanji, and in real names Zoro-ya, Nami-ya and Nico-ya.

Ya are translated as Mr. or Miss. in English. But in Japanese adding ya defines someone's profession. It is not that Law does not use ~-san at all, Like with Corazon and sarcastically with Vergo. As stated in this page.

So why does Law use ya rather then ~-san, ~-kun, etc. while addressing others?
From the SBS (質問を募集する, &quot;I'm Taking Questions&quot;) question and answer column of volume 62, chapter 609, page 122:

D: Like &quot;Mugiwara-ya&quot;, Law-san tends to call people with the &quot;-ya&quot; suffix, but in the case where the person's last name is &quot;Tsuchiya&quot;, then does that become &quot;Tsuchiya-ya&quot;? Please tell me Law-san♡ P.N. HeartLoveWoman
O: Mr. Law—they're calling you --! ............. Aw he won't come-. Let me answer instead. A long time ago, around maybe the Edo period--. There was this thing called &quot;Yagō&quot;. So if for instance, in the case where there were two Mr. commoners named &quot;Tomekichi-san&quot;, things like &quot;Dōgu-ya no Tomekichi&quot; (Tomekichi of the Tool shop) or &quot;Oke-ya no Tomekichi&quot; (Tomekichi of the Bathtub maker); something with &quot;~ya&quot; would be used in place of a last name-. Like how you shout &quot;Tamaya~&quot; at the fireworks or &quot;Nakamuraya&quot; from Kabuki; have you heard of those? In other words, he just goes with the flow. Right, Law-san?
L: Yeah.

&quot;D&quot; denotes the reader (dokusha), &quot;O&quot; denotes Oda, the author, &quot;L&quot; denotes Law.
Essentially it's as as if Law is calling Luffy &quot;Strawhatter&quot; or &quot;Strawhat guy.&quot; Like you would refer to a butcher (nikuya) or a baker (panya) by the shop they run. Note that no honorific is applied, so you cannot assume this to be polite. You could say it puts a bit if the distance between the speaker and the one they are speaking to. Much like in a MMO party where someone calls you by your class or your role instead of your character name.