Why Haku from Naruto looks like a girl is a question that has plagued the Naruto fandom for quite some time. Bishonen finds its traces through multiple anime since its conception, though literary traces of this can be seen as far back as 1603.
In Naruto, Haku is a bishonen
Feminine-looking boys are not a new staple in the media, yet their prevalence in modern societies has been steeped in years of art and entertainment. The categories of men and boys have been separated in preference and attraction.
Attraction is a subjective concept that has existed for all time, yet an expansive and popular beauty standard has always been prevalent. However, the opposite can gain traction as well.
The words feminine and men are considered contradictory as it is assumed that masculinity and femininity occur between the opposite genders. A feminine male is a direct contradiction to the typical war-like masculine trope that most societies love.
The answer to why a male character seems feminine may be in line with the annals of history. In 1603, old Japan, Izumo no Okuni, created the first Kabuki theater.
She recruited female dancers to participate in her performances. Surprisingly, their shows became quite popular and became influential in the grand scheme of things. Rival groups soon formed and it had a massive and cultural impact.
However, the ruling shogunate disagreed heavily with the art due to various reasons. One reason could be that most performers were participating in the oldest profession after they were done performing.
Soon, all female performers were banned, so male actors took over their roles. However, this brought about the same issues. The male performers who participated in the theater were chosen for their feminine features.
These were called the wakashu. Young boys were preferred to play feminine roles as they resembled their female counterparts. They were soon banned as well, so now feminine-looking young men became the performers. Despite the drawbacks, the Kabuki theater is still massively popular.
The term bishonen is partially inspired by the wakashu, though other factors contribute to the term. Bishonen is a feminine and gentle-looking young man who happens to retain masculine traits. Their skin is fair and their hair is usually styled in an attractive manner.
Haku in Naruto is a fifteen year old boy who seems to be based on a bishonen. Haku is fairly skinned, with black hair styled differently from other characters. He doesn't appear physically athletic, but he has an amazing amount of stamina that only Sasuke can surpass.
Another aspect of the bishonen that he possesses would be his Kekkei Genkai. Condemned for his bloodline and his ability to manipulate water, though his talents truly shine when he activates the Demonic Mirroring Ice Crystals.
This ability allows him to travel through mirrors in rapid succession, though it appears as if he's in every mirror at all times. Sasuke Uchiha tried to detect him but couldn't until he activated his Sharingan.
When Naruto first encounters him, he's portrayed as a sensitive boy who poses no harm to others. They had a meaningful conversation that further established this fact and they part ways.
This part of the narrative is supposed to establish Haku as non-threatening, so when his true self is revealed, it is supposed to be a plot twist.
By this point in the show, the layers of wonder and ninjutsu had been slowly peeling away to reveal a world of false hope and mercy. However, even when Haku was killed, Naruto showed mercy and buried him next to his master.
In conclusion, Haku is a product of the bishonen genre that happens to bleed into the cracks of anime and art. He was supposed to appear as a quiet, feeble bird when really he had sharp talons all along.