UUUUUUUUH Sorry BlackCat that's actually a little bit off. TIME TO HIBIKI'S GUIDE TO JAPANESE CULTURE AND LANGUAGE!

First things first, for the most part, in Japan, calling people by their last names is a very ordinary custom and something that basically nearly everyone does. So instead of calling Shinji, you would call him Smith. Honorifics are what denote what you feel about the person, your realtionship level and even bits of your personality. 

Alright for honorifics and such it goes like this...
[list]
[*]"-san" is the go-to honorific in unfamiliar situations, it basically the Japanese equivalent of "Miss" or "Mister". Used to show politeness and respect, it is used with people at the same social station, level or class as you and when you need to be polite. Is best used in conjunction with the person's last name, example being, "Kunazumi-san".
[*]"-chan" is the honorific used on the end of people's names to show affection or attachment to someone, usually a female, friend or lover. It's basically the same as calling, "Timothy" as "Timmy". It can be rude however to do so with people you've just met as it is rather intimate. It's also used in conjunction with nicknames for example, "Kuyu-chan", "Lila-chan", "Sumiko-chan" or even "Theo-chan".
[*]"-kun" is generally used with boys but can be used with anyone with a lower socal status or lower power than you. When used with peers, it's generally a case of familiarity and endearment. When addressing someone of a lower status, its used to avoid the intimacy or inappropriateness of using "-chan", where it may be deemed so, such as a teacher talking to a student. Example being, "David-kun", "Kikuyu-kun" and "Theodora-kun".
[*]"-sama" is basically the equivalent of "Master/Mistress" and are used to people who are of much, much, higher status then you. It's used to show just how much respect you have for the person your addressing and also sometimes used to address customers of a buisness. Example being, "Lilacflour-sama".
[*]"-senpai" is basically "senior". You use it to address people who have more experience than you but not by too much, such as upperclassmen in your high school or in the bakery's case, people who have worked here before you. Example is calling the twins by, "Kunazumi-senpai" as they are the very first two employees.
[/list] 

That is how you generally use honorifics in Japanese culture. There are more I can list but those are the main ones. Always try to keep this in mind when addressing people in the RP as for the most part, they will be a big part of the whole Japan feel.