Yobitsute [ 呼び捨て ] means "Addressing someone without a -san or other honorific title
", in japanese culture having the proper conduct when speaking is something that is highly endorsed to everyone from early age, and part of that conduct is addressing people's name in a respectful manner; in fact rules are usually so strict that you aren't supposed to call people by their names because it "sound too personal" but rather to common acquaintances you need to call them by their last names and adding an honorific as to not sound disrespectful; "inversedly" calling people by their names and without honorifics is a proof of intimacy, for example a mother would call her son by her name while they are in home, but if that same mother and son actually have a teacher and student relationship on school then the mother is likely to use his last name + honorific to call him out while in class and school, since the relationship that a teacher and a student have isn't supposed to be one of close intimacy even with the exception of being mother and son; that's due to work ethics and a bunch of other stuff.
Let's watch a image heavy long graphical example by following one of the usual spats between the good old "Kitsune" (referred as "Y-san" in the translation) and "Niku" (referred as "S-san" in the translation); there is also wanna be delinquent referred as "K-san", but he is not relevant anyway.... Rather than a correlative order, I choose the pages that illustrate the point of "yobisute" the most.
Y-san : And your name?
K-san : Hasegawa Kodaka
Y-san : Kodaka, hm....
Y-san : You have absolutely no grounds to talk to me about friends, Kodaka
K-san : ( That was [Yobisube] )
Y-san : ?
Y-san : What is it?
K-san : It's nothing, don't worry about it.
The image depict the first direct conversation between K-san & Y-san, even though they have been in the same class for a while they haven't actually ever speak to each other and in this first meeting the topic that neither of them 'has any friends' popped up (thus, the reason why Y-san says "You have absolutely no grounds to talk to me about friends, Kodaka" , since he
was pointing out that she wasn't being friendly with anyone, Y-san retorted that his case was the same).
Yobisute is not a normal attitude for someone who you speak with for the first time, thus why K-san was surprised by being called "Kodaka" (his first name) and starts getting embarrassed in the next panel, in fact the text over his head reads something like "he is happy about being called by his name so casually". Next example is "the other way around".
This conversation takes place the next day, and Y-san has been calling K-san Kodaka for a while, but he hasn't "done the same" (gone yobisute) due to them not being too intimate with each other.
Y-san : ..... There is something that has been pestering me for a while.
Y-san : That "you and you" that you have been going around, drop that, it's getting annoying
K-san : Ee? aa.... I get it....
K-san : Well.... eeerr.... er....
K-san : Mikazuki-sa-----
Y-san : Yozora
Y-san : Just use [Yobisute] and use Yozora
K-san : ..... I- I get it
K-san : ..... Yo- ..... Yozora......
Y-san : Why do you go blushing like that? That's simply repulsive
K-san : Don't you have some sort of nick name? or something to call you by instead....
In these two pages, after been acquaintanced for a while, K-san has been calling Y-san "omae" (you), not only respecting the "not to yobisute" rule, but also kinda embarrassed that he is speaking with a girl as he has little experience with people; at the point that the pages show Y-san has gotten annoyed of being called "you you" up and down by Kodaka and got irritated by his shy attitude, thus she pushed him to use yobisute on herself; not understanding that he has reasons to be embarrassed about since she is a KY (someone who doesn't care about reading the atmosphere). There are reasons of why Y-san pushed K-san into yobisute, but you'll have to read a lot of chapters of this to find out (and no, it's not that she liked him from second one or something like that).
S-san : Well, with this well leave it for today
S-san : Also, that Yankee over here, just go ahead and call me Sena
K-san : And why should I?
S-san : If you go around calling that Kitsune by her name, but address me by my last name; wouldn't that give the impression that you are putting her in higher value than me?
K-san : ....... I understand .............. Sena
In the current scene S-san intruded in the situation, even though it was the first time that Y-san spoke with S-san things clicked the wrong way right away and they got into an argument and worse than that from second 1. As someone who refuses to lose, S-san pushed K-san to use yobisute on her, else it gives the impression that Y-san has a superior status as "a friend" in K-san's view. Normally that's not something you get mad about, and having a "superior status" is usually given by using important honorifics, but being someone that hates to lose, S-san saw things the other way around since K-san was definitely not
going to use high praise honorifics than her, thus it was giving the impression that he was being distant to her and a close friend with Y-san. Something similar even happened in KoF, with Kim having his "Kim team", and Jhun not being able to stand him getting that recognition and eventually made a "Jhun team" for 2002UM, the team example is not yobisute, but rather an example of "hates to lose" even when it is about small things.
BTW, K-san is called Yankee, the usual word for "delinquent" in japanese since he is blonde, and people assume that he has dyed his hair to be a delinquent, rather than being someone who has japanese features yet was naturally born blond (which is the case).
Now, as for KoF, there are characters that follow a respectful speech structure such as Athena and pretty much call everyone with honorifics, even Kensou who she has a close and long relationship with; other characters that are closer to each other simply go yobisute and address themselves by their names (such as the previously explained childhood friend case of Mai and Andy).
There is also the "other side of the coin", where someone goes yobisute for the sake of lacking respect towards the target; for example since honorifics are "that", a way of bestowing and showing respect between users
, when there is hate and no need for respect, yobisute is used or rather "lack of honorifics
". Kyo and Iori go yobisute with each other and they don't do it out of love; there is also the K9999 and Kanedanagi intro from 2002, which played the yobisute card straight with K9999 using youbisute out of lack of respect, and Kanedanagi demanding respect from the brat, which of course comes as a carbon copy from a mythical legendary scene in Akira.