Why is Kaiji referred to by 1st name while Akagi is referred to by last name?

Question

Kaiji and Akagi similarities

1. They are both gambling manga series written by Nobuyuki Fukumoto.

2. They were both adapted into anime series and live action series/movies.

3. They both have a single word in the title which is the name that the title character is often called.

4. In the anime adaptation, both characters are voiced by Masato Hagiwara, and both main bosses of s1 are voiced by Masane Tsukayama (Iwao Washizu, Kazutaka Hyōdō).

5. Both series have a narrator.

Given such similarities, I am inquiring about the following difference I notice:

Question: Why is Kaiji Itō (伊藤 開司, Itō Kaiji) often referred to with 1st name Kaiji, by the narrator, the title of the show and the characters while Shigeru Akagi (赤木 しげる, Akagi Shigeru) is often referred to with last name Akagi (also by the narrator, the title of the show and the characters)?

Guess: What I have in mind is there's something specific here like, say, age of the characters at the start. But in this case, I think it'd more the reverse:

1. Akagi is around 13 yo at the start, much younger than the other characters who are dirty cops or yakuza, while

2. Kaiji is like 20+ yo, around the same age as some characters. And while still much younger than the Teiai goons, Kaiji is still called Kaiji, Kaiji-kun or Kaiji-san by pretty much every character regardless of their age.

Answer 1

From reddit user Arcengal in here:

I don't have a definitive answer, but I think the most likely one is it's meant to show the contrasting personality between the two characters.

Kaiji is a bum and he makes poor decisions in life, but at his heart he's very personable and almost too open to other people. The author shows this by how relaxed he is with other people using his first name in conversation instead of his family name, even relative strangers ("Kaiji-kun!"). This isn't the standard for Japanese culture - 'first name basis' is serious business. In terms of other characters using his first name (Tonegawa etc.), this may as much false endearment on their part as it is the author sticking to an established name convention, but overall the Kaiji manga is more about emotional responses to stressful situations, compared to the clinical play patterns of Akagi's.

Akagi, meanwhile, has a lot going on in his head, most of which we aren't privy to. He doesn't overly care about close connections and always keeps people at a distance, so people are more formal around him and never feel close enough to switch to first name. It's not as if he's shy about giving his full name (he does so at least twice in the anime), but he's also the same guy who kneecapped someone then went off to Mahjong against a blind dude. Would you want to get too close to someone like that?

From reddit user CactusSnail in here:

Kaiji's very "friendly" and frequently gains and loses comrades throughout his gambles whereas Akagi flat out tells Osamu that he doesn't want him to think they're friends just because they spend time together.

Kaiji is a loser, barely scraping by a lot of the time and not having any real ambition, struggling every step of the way to get out of whatever mess he's in. While Akagi constantly succeeds and commands respect from those around him, actively choosing to discard wealth and power he's gained through his gambles to keep himself free of it.

Socially they're practically opposites, and FKMT reflected that in how they're addressed.

Note: FKMT is Fukumoto.