OKAY I IMMEDIATELY LOOKED UP A JAPANESE LP TO CHECK FOR MYSELF BECAUSE THAT’S THE KIND OF PERSON I AM and here is what I learned from skipping around. Note that I’m not an expert in Japanese by any means! I can just read bits of it, so I could be wrong about connotations. SPOILERS!
FIRST TO ANSWER THE OP’S QUESTIONS
Sans refers to Papyrus just by his name, with nothing attached to it. He does refer to him just once as otouto - little brother - when he’s telling you about how Papyrus is a human hunting fanatic when you first meet him. Pretty standard for older siblings, as far as I know.
Papyrus in comparison always uses nii-chan for Sans which is an affectionate, familiar, casual (kind of cutesy?) way to address your older brother. DESPITE THIS THEY COULD STILL BE TWINS because apparently in Japan even twins make a note of who got technically born first, even if they were born at practically the same time. LOOK YOU’LL TAKE THE BROTHERS BEING TWINS FROM MY COLD DEAD HANDS OKAY. He does use Sans’s name once when he’s all “the next puzzle will be by my brother, Sans!” but otherwise he almost always uses nii-chan when he talks about him. On some occasions he drags it out so its nii-cha~n but I couldn’t tell you the significance of that, haha.
Undyne uses watashi for herself. A straight-forward neutral pronoun.
Papyrus uses ore-sama for himself, as the reply up there mentions, and on one or two occasions I saw him drop down to ore. I only saw him use boku once, when he’s asking Sans where his action figure is. Otherwise, as a Lost Soul, in his notes to Sans, when he’s showing you his house, before during and after his date, in the phonecalls, in the epilogue, when he’s dying in the murder run, and when he’s chatting with Sans in Snowdin before he notices you’re there, he always uses ore-sama (or on some occasions, Papyrus-sama lol). So it looks like Papyrus only uses boku with Sans, and only when they’re at home alone.
Mettaton uses boku. During the musical though, he switches to watashi, but goes back to boku. Also uses boku as Mettaton EX and NEO.
The Fallen Child uses watashi, but they use the kanji for it. Asgore and Toriel also use watashi, but they spell it out in hiragana. The Fallen Child actually uses a ton of kanji when they speak, way way way more than anyone else in the game. It’s really jarring. (The narration does not use kanji in general, including the red text.) Mato wrote about EarthBound suddenly using kanji in a similar way that could be relevant. In the EarthBound book, Mato says Itoi put kanji during that cutscene to forcibly pull the player out of the game in a surprising, memorable way, and to remind the player that they and Ness (the main character) were not the same person. Your existence as a real person outside the game playing it is very important in EarthBound.
Flowey and Asriel got covered above, haha.
SO NOW FOR OTHER THINGS I NOTICED
Alphys also uses watashi. Aaron uses boku. Royal Guards 01 and 02 use ore or maybe orera (I wish I could tell you more about their speech patterns but that’s way above my skill level). Monster Kid (who talks like a punk kid) and Mad Dummy use ore. I didn’t catch Napstablook using a pronoun. Mettaton does call “Bloo-!” at the end of his battle, so the nickname is probably the same.
Papyrus writes his text vertically (although, when he says “cool” in english it’s sideways teehee). He also has his own font but it’s not as obvious. Sans’s font is hideous lol. Papyrus mentions fonts when he talks about being trolled online with puns.
Papyrus’s laugh has been changed from Nyeh Heh Heh to Nya Ha Ha. They also changed “Wowie” to “U-hyo-u”.
Alphys’s name lost the s sound for whatever reason, so it’s just Alphy.
Temmie Village is FASCINATING and I wish I knew more about Japanese so I could really get it on all levels, but as far as I can tell, the Temmies randomly mix hiragana and katakana in words, sometimes add english letters, and their name (Temmie) is written in kanji. They still go HOI and Bob is still Bob. The temmie flake description mentions rice I think so maybe it’s a furikake joke of some sort? When they say bye they go “byenara” which I assume is a mix of bye and sayonara.
In battle Temmie’s introduced as “hidden boss” Temmie and (I think) says she’s frightening in a cutesy way. Her check dialogue is in Temmie speak. Most of the Temmies in Temmie village refer to themselves as Temmie, but in battle she uses “achishi” (a corruption of atashi?) and adds nya in words.
Almost nobody uses an honorific for each other. There were two exceptions I saw - when Papyrus is reporting to Undyne in Waterfall for the first time, he calls her Undyne-taichou (although in all other cases, he just calls her Undyne), and when Toriel and Sans first meet in the Pacificist ending and he introduces her to Papyrus, she calls him Papyrus-san (and otouto-san). I didn’t see her use honorifics for anyone else though. So Papyrus is special. :3
The first part of the wrong number call “Hello, can I speak to G…” is in English with English letters, then it goes back into Japanese.
Instead of a Halloween party I think the brothers went to a cosplay party.
The word search got translated! :O I didn’t even think about that. Sans mentions crosswords but Papyrus talks about some other game than Junior Jumble that I couldn’t catch.
Sans uses anta and omae with Papyrus, which are really casual and familiar forms of you (but he uses them with others too, though). Papyrus uses kisama a lot for you the human and in general.
When Papyrus calls you he goes “moshi moshi! Papyrus desu”.
Alphys calls Asgore Asgore-sama at one point, though it’s usually King Asgore, and she uses a lot of Japanese emoticons in her Undernet posts. Both Papyrus and Napstablook’s screennames are written with English letters.
Mettaton goes OOOOH YESSSS in English with English letters, and he likes throwing in English words written in katakana in his speech a lot. He refers to you and I think the audience in general with kimi (or suteki no minna-san). Napstablook also uses kimi for Mettaton.
Mew Mew Kissy Cutie is Kiss Kiss Cutie Mew Mew. Mew Mew Kissy Cutie is Kiss Mew 2, haha.
Catty and Bratty use a lot of ~~s and atashi for themselves, no surprise there. They talk very girlishly but I think the valley girl nuance they had got kind of lost.
As a Lost Soul, Sans drops oira for himself and uses ore instead. It looks like he also switches to ore by the last corridor in the murder run. Instead of MS Sans Serif, it looks like he just uses the game’s normal font whenever he drops out of Comic Sans.
Asgore uses kimi for you and Toriel, but he writes it in hiragana, while most of the others I saw who use it write it in katakana. Toriel uses anata for you and Asgore instead. Asgore still calls her Tori though, and so does Sans (he also calls her obasan when hes’s all “sorry old lady” in the murder run). Toriel used to call Asgore Goa-chan (Asgore -> Azugoa). Alphys calls her fanfic “TORI x GOA”.
Ningen - human - is almost always written in katakana, presumably to emphasize the word.
Asgore goes “hello” in katakana phonetically instead of howdy I think. So does Flowey.
I don’t really know enough to say if Papyrus has weird grammar at points like he does in English, but the little noises he makes seem kind of strange and tend to trip up the narrators. Like at one point he laughs backwards (Ha ha ha ha hya ni!).
Snowdin is called Snowful. The Librarby’s sign is still misspelled, with toushonka instead of toushoukan. Lemonbread’s name is written in English letters as LEMON PAN. The Memoryheads use kimi for you. Popato chisps are the same (popeto chissupu). Chairiel is the same.
Gerson talks like an old man, no surprise. King Fluffybuns is Fuwarin ou.
The macaroni picture says “Papa King” and the sweater says “Mr. Papa” (misuta papa).
Now, the mirrors are interesting, and I could be missing something here. But during a normal route, the mirror says “jibun da!”, in Asgore’s house it says “iroiro attakedo jibun wa yappari jibun da.”, post-pacifist it says “yappari jibun da yo Frisk.”, and in the murder run it says “kore wa jibun… [player name] da.” I don’t know enough Japanese to really tell you how close it gets to the “it’s me/it’s you” shift.
You can put in papirusu in kana and it’ll fit, and Papyrus will still let you use it. Undyne still calls you a punk (panku) for putting in her name. Putting in kyara in katakana OR the name in English letters still gets you hontou no namae (real/true name). The word for “character” in Japanese is kyarakuta in katakana.
This next thing has some layers to it so bear with me. When Itoi was making the first Mother game for the NES, he made a deliberate choice not to use kanji and only use hiragana/katakana for the text, making it sound more like spoken Japanese and encouraging players to read it out loud as they played. He kept this decision for Mother 2/EarthBound and Mother 3, giving their text a unique feel in Japanese. As is common knowledge by now I assume, Toby spent A LOT of time in the EarthBound fan community and Undertale owes a huge debt to EarthBound in many ways. Undertale’s Japanese localization DOES have kanji in it at times (notably in signs, the Fallen Child’s dialogue, Temmie’s name, and the kanji for brother and king and mountain)… but the vast majority of the text I’ve seen is just written in hiragana/katakana, like in the Mother series. I wonder if that was deliberate.
That was all I picked up anyhow as I skimmed around. Anyway, I’ll put some caps I took of this stuff under the cut if for some reason you want to see, haha.
