@Solano
then what is the point of his participation to the BAN...
I think it was to defeat Lalo, but in a safer way. Because of how he set it up, he got a few advantages:
-He made Lalo bet everything in this match. Not just his life, but all his assets. (interestingly, he didn't make Baku do the same)
-Because he's the leader, Lalo's team won't try to kill him during the match. (which he tried to abuse against Fukurou)
-He got a strong gambler (Baku) to face Lalo for him. He learned that Baku played Surpassing the Leader with him before (from Lalo's phone call), so he can trust his skill.
-Even if Baku loses, Hal would get first-hand experience in how Lalo fights and thinks, so he'll be better off during an actual StL match vs Lalo.
he had not any possibility to know that the BAN he imposed would have him participating and with Baku choosing him.
The fact that it was a BAN match significantly increased the chance of allies participating. It encourages you to add allies with the "men" card, and Hal knew that Lalo has talented people like Voja under him, so he'd be tempted to bring allies. Hal was the one who decided to make it a BAN match too, which leads me to think he did it on purpose so he could join the match as an ally.
As for Baku choosing him, I can think of two possibilities:
1) They agreed beforehand to be allies. In between the time Hal and Baku met in the bookstore, and Lalo came for the meeting, they had a off-screen discussion beforehand and plotted this plan against Lalo.
2) They didn't discuss anything at all, Hal just gambled on it by making it a BAN match. He probably assumed that Baku has few allies because he came alone to the bookstore, and that Lalo has a lot based on the conversation he had with him.
I don't think Baku offering for Hal to be his ally was just a spontaneous offer that he decided to accept. We know that Hal is smart enough to calculate very far ahead, so I definitely think he predicted and wanted for this to happen.
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Your question about Hal's motives is very good. I don't think my answer is perfect, it does feel like Hal has some other motives that haven't been fully explained. Like why he continues to work with Gakuto, and also why he promised Nobuko that they'll meet again. And also how he acted in the past with Baku (which I mentioned in my comment to Veshv). Definitely feels like there's more to it.