@thadzkie20 We don't know why yet, but his dad is probably either a descendant or reincarnation of one of the original Breath of the Sun users we've been shown before:
Two of them look almost identical, probably twins or at least brothers. One of them is Upper Moon One, but which one? Gotouge has often used small character details, like the pattern of Sabito and Tomioka's kimonos or the magatama tied around Zenitsu's senpai's neck, to draw connections in the past, so let's try to use their different appearances to figure it out.
- First and second images: one of them has a white top and white belt, slightly flatter hair, a sword with a wrapped hilt and cross-shaped guard, and a mark extending below their left eye (and not onto their neck like Upper Moon One). The other has a, uh, screentoned top, hair that sticks up a bit, a sword with a bare hilt and cross-shaped guard, and a mark that does
not extend below their left eye.
- Third image: The one who almost killed Muzan had a screentoned top and what looks like a white belt on, pointy/poofy bangs, a bare-hilted sword with a cross-shaped guard, and a mark extending below their left eye.
- Fourth and fifth images: The one in Tanjiro's ancestor-dream had a screentoned top with a white top underneath it and a white belt, sparser bangs and kinda-flat hair, a bare-hilted sword with a
square guard, and a mark extending below their left eye.
- Sixth and seventh images: Upper Moon One has a snakeskin-ish top and a white belt, long bangs that stick up a lot, a sword with gross eyeballs on the hilt and a cross-shaped guard (which also has eyeballs on it), and a mark extending below his left eye and rising up the right side of his neck.
- Eighth and ninth images: The training doll based on one of the swordsmen has a screentoned top and white belt, hair and bangs that stick up a bit, bare-hilted swords with cross-shaped guards, and no mark on its neck (apparent in other pages from chapter 104). The sword that was hidden inside it has a wrapped hilt and an
oval-shaped guard.
Going through the various clues:
- The guy in the screentoned top shows up a lot, but his bangs and hair aren't always drawn the same way. Could be an art mistake.
- Upper Moon One's top is different from either of them. He's got the white belt that they both seem to have, though.
- Upper Moon One has way longer bangs than either of the two, but his hair is generally spiky, like screentone-top guy's seems to be in the group shot of the Sun users.
- The bare-hilted sword shows up in every appearance of the guy with the screentoned top. Interestingly, the sword hidden in the doll isn't the one used by the guy who almost killed Muzan. Maybe it was made after they failed to defeat him?
- The mark on the screentoned top guy doesn't extend below his left eye in the group shot of the Sun users, but it does in every appearance after that. Ergo, the group shot is probably the earliest chronologically.
- Upper Moon One has a mark on the right side of his neck, but neither of the Sun users seem to have one. However, there are two instances of a neck being conveniently obscured: once in the group shot of the Sun users, and once in the shot of the guy who almost killed Muzan. Both are ostensibly shots of the screentoned top guy.
In conclusion, Upper Moon One is probably the guy in the white top, but the other guy's hair is drawn kind of inconsistently and it's giving me doubts. Maaaybe the screentone-top guy seen in the group shot and Muzan's memory is Upper Moon One and his brother took on his outfit and sword after his defeat? Those two shots show him with spiky hair/bangs
and obscure his neck, so it's not impossible...