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As mentioned in my article on kanji, I just wanted to write a small article about slang you might encounter in manga or in anime. Obviously, I won't explain everything there is, because, as I said, since manga and anime are entertainment, people take great liberties when it comes to the language, so what you see in one manga might not be the same in another one. For example, one author might use katakana for some words that would normally be written in hiragana, whereas another author might not do that.
Sometimes, the choice of language chosen by the author might be attributed to one character to give this character more personality. So, it all depends on the author, so the language might vary from manga to manga, or even from character to character.
As I said, the way they speak in manga or anime is not exactly how we speak in everyday life, so you have to be careful with that. In manga and anime, you will see the casual form way more, as well as slang terms, and in manga and anime, this kind of language is not necessarily deemed rude, whereas if you would speak that way in Japan, it would be considered rude. So, the kind of language they put on in manga and anime really should be used with your closest friends.
So, in manga, one thing that you might encounter is the usage of katakana for words that would normally be written in hiragana. For instance, you might see the word これ written as コレ. There is no reason for this, other than the author deciding to write this way rather than that way.
You might also encounter the use of the small vowels. For example, to make a vowel sound longer, you will see this さくらですよぉぉぉ rather than using the う to make the 'o' sound longer. You might also see it that way with katakana instead さくらですよォォォ. You might also see it that way さくらですよー. We will use the ー in katakana for long vowels.
You might also encounter the mini つ in brutal stops, like さくらっ！We use it this way to indicate that the character made a brutal stop or that there was an interruption. なにっ？
I will give you some examples for slang that you might encounter, but keep in mind that this is not an exhaustive list.
わかんね: this is the slang term for わからない (I don't understand). The ら turns into ん, and ない turns into ね. You might also see it written like this わかんねぇ. Other verbs will be written this way, such as ふざけんね (don't fool around!). In this case, the meaning is 'don't fool around' so this slang is the slang term for ふざけるな 'don't fool around'.
すまんすまん: this is the slang term for すみません.
やばい: this is the equivalent of 'oh my God' or 'oh no'.
ダサい: it means 'lame' as a fashion sense or a person's personality.
キモい: it comes from きもちわるい which means 'feeling sick' or 'gross'.
ズルい: it means 'unfair' or 'you're cheating'.
ムカつく: it means 'irritating' and it comes from the term むかむか (piss off).
ウケる: it means 'interesting' but not in a good way. It is used to mean 'laughable'.
ウザい: it means 'annoying' but its use is harsher than the English equivalent.
ダルい: it means 'lazy'.
げき: it is a term that means 'very'.
マジで: it means 'seriously' as in when you are surprised 'seriously?' 'really?'
おしゃれ: it means 'fashionable', someone who has style.
おっす: it means 'hi' if you don't want to be too formal. It is used mostly by guys.
かまちょ: it means 'clingy', someone who does not give people space.
ブスカワ: it means 'ugly-cute', like pugs are ブスカワ.
イケメン: it means a boy with a pretty face. It comes from いけてる (cool) and めん (face).
オタク: yes, it is a slang, and it means someone who is too much into anime, video games, or manga.
カッケー: this is the slang term for かっこいい (cool).
ちょうしどう？: this is a slang term that means 'what's up?' or 'how are you doing?'
ケーワイ or KY: it comes from くうきよめない and it means someone who cannot read the mood.
それな: it means 'I know right' or 'ikr'.
wwww or 笑 (わら): this is more used while texting and it means 'lol'.
おし: it means a person who is a fan of something, so it means 'fan'.
You might use any of these with your friends.
Like in English, slang have a tendency to disappear or change over the years. Some new ones appear, so sometimes, we cannot always be up-to-date with them.