Two great moments: Baku's face when NeroNero proposes a gamble for the King's spot, and trying to light up a cigar in Kyara's honor.
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Veshv
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Woodchipper
Yes, Nowa told Kaji that he betrayed an ally to save himself very early on in the manga. Chapter 33 to be exact. It's come up a few times, and notably recently in Chapter 316 during Kaji's dream.
Although we know Hal didn't actually die in the match vs Fukurou, Baku tells Yuukiide that he "killed" Hal. In that gamble with Fukurou, Hal says he is betting his life as Hal. https://mangadex.cc/chapter/713885/12 Whatever happened that day, Hal lost (and Eba was killed), and it seems to have fundamentally changed Hal into the current form of Souichi/Hachina, so its likely that Baku feels a sense of guilt for the loss of his friend. Baku tells the story in Chapter 90. https://mangadex.cc/chapter/204647/1
The weird thing about this is that Baku says he had a gamble set up that day and Hal went in his place, and lost. However the flashback seems to imply Hal set the gamble up on his own and posed as Baku while setting it up. I'm not really sure where the truth is here. Either way, Yukiide tells Baku that it's not really his fault since it was Hal's decision to do so, but Baku blames himself anyway.
I think its very likely that the rumor that got back to Nowa simply stated that an ally of Usogui went to the gamble in place of Baku and died in a russian roulette style game. In the flashback, Hal had set the game up without Baku's knowledge, and Eba pulled a trick to make it look like Hal died in the match. I don't think all the details of this game are fully understood yet. How Fukurou could have beaten Hal in a game of wits is beyond me, and I'm assuming we'll get more information on that and how he (probably) cheated. He's the last ally of Lalo left, and it certainly won't be Marco taking him on, so Hal will likely get a rematch.
As far as we know Kyara is the first ally of Baku's to truly die in a gamble for Baku's sake, and its something Baku has really tried hard to prevent. Remember, he gave up a potential winning turn in the labyrinth to try and revive Marco.