Zeno thanks Hak as "the four dragons' delegate". It seems like Zeno being silly and the scene is played for laughs but since he is the oldest in the entire group he has reasons to call himself that.
In Chapter 76, Zeno brings an old woman to the group's merchandise, calling her a "young lady" as he does. Every woman is "young" compared to him.
In chapter 102, Zeno mentions that he'd been aware of Yona for a while and actually watched her leave the castle and journey to find the other dragons. He never intervened in anything because he was testing her to see what she would do with the dragons' power. He joined right after the Awa arc. Why? Because it was the first time Yona led her group to help other people.
And he recognized Tae Jun in that village because he saw him when he was chasing Yona and Hak or even before (when Tae Jun visited the castle to woo Yona). The same with "the faint smell of death" on Hak: he has already seen how far Hak will go to protect Yona. Another example is when Zeno was surprised to see Soo-Won in chapter 88: some thought this was because he recognised him as Hiryuu's reincarnation, but the truth is that he simply recognised him because he had been watching Yona.
Remember how much power the priests had before they were driven out of the palace? They likely had that power because the first priest was Zeno, the Ouryuu.
Yet another Zeno one. In chapter 97, while Zeno's taking care of the sick dragons, Kija comments that it feels nostalgic. Turns out that Zeno had secretly visited the other dragons while they were children. Kija, having been four at the time, vaguely remembers Zeno's visit.
By Seiryuu Ao's lifetime, no one in the village remembered the story of the four dragons. Ao and Shin-ah and the villages believed the dragon's eyes were an inexplicable curse. But compare them to the Ryokuryuu villagers: Jaeha knew everything about the four dragons, even though he was just as oppressed and ostracized as Shin-ah. The difference comes to light in the "Lushing Forest" arc. The Seiryuu who possessed Shin-ah knew about the four dragons, and was able to identify Zeno as Ouryuu, but he died when his successor was just a baby. That little Seiryuu had no one tell him his history. Even if the villagers knew about Hiryuu and the four dragons, they likely avoided the little Seiryuu and ostracized him, so whatever information he did have was spotty and incomplete. Generations later, less and less of that information is remembered and passed on. Losing that one vital link in the chain resulted in Seiryuu village losing all of their history.
King Il refused to make Su Won his successor despite the latter's obvious talents because he would have had a declaration of war from Xing at his doorstep otherwise, given Yu-Hon's war crimes.
In chapter 1, Soo-won mentions to Yona that he's had some discussions about marriage before. In chapter 135, it's revealed that Joon-ji has been trying to get Soo-won to consider Lili as a marriage candidate for quite some time, making her likely one of the subjects of the marriage discussion he was talking about.
When Jae-Ha drinks the hallucinogen Nadai, he passes out quickly afterwards and has to stay in bed for most of the next day. In comparison, the addicts that the heroes fight are sound enough in body to fight. Jae-Ha drank alcohol mixed with Nadai; mixing drugs and alcohol is not recommended because it can strengthen the effects unexpectedly.
The Wind Tribe has notably fewer adult residents and many orphans in its population. Later, it's revealed that Kouka had a particularly bad war with the neighbouring country Xing 17 years prior to the plot, of which the Wind Tribe is situated closest to than all other tribes. Because of this, it makes sense that they would have lost many young men and women to that war, leaving behind a sizable gap in the population and many orphaned children. One of these children, as confirmed by Word of God, being Hak.
This also explains how Hak became the youngest general in Koukan history, as both of Mundeok's sons died in that same war. Mundeok lost both of his heirs at the same time, and had to raise a new one basically from toddlerhood.
