Warning: contains spoilers for Hunter x Hunter
Hunter x Hunter is famous for its rich characters, and with a prominent character being transgender, the manga is already far ahead of Marvel and DC's transgender representation. More than just being a one-note side character though, this character is deep and nuanced in a way that is rare to see in mainstream manga.
Though much of Yoshihiro Togashi's Hunter x Hunter focusses on the adventures of protagonist Gon Freecss, part of the manga's appeal is in its wider cast. Gon's best friend Killua Zoldyck in particular receives a great deal of development as his time with Gon helps him break free of his cold family of assassins. After Gon is seriously injured though, Killua returns to his family to retrieve the one person who can save Gon's life, his little sister Alluka. While Killua exclusively refers to her using female pronouns, most of their family isn't so kind, frequently misgendering her. Making matters worse, she has an alternate personality named Nanika who is one of the series' most powerful characters.
Nanika is able to grant any wish upon the completion of four requests. However, the greater the wish, the greater the requests will be. If all four wishes are rejected, then Nanika uses her immense power to instantly murder whoever refused along with their greatest loved ones. As a result of this, Alluka's family locked her away deep within the depths of their mansion, and even go so far as saying that she isn't part of their family at all. Killua is the only Zoldyck who views her as a person instead of a monster or a tool to be used. While her story isn't about her trans identity, it's easy to see how many transgender readers could relate to the feelings of familial abandonment and isolation Alluka experiences throughout the story.
What makes Alluka such a forward-thinking character is her prominence in the plot. Alluka is more than just a side character, she's the focal point of an entire story arc in the manga. Even more remarkable though, is that Alluka isn't even Hunter x Hunter's only LGBTQ character. The villainous Chimera Ant Neferpitou is presented as a man, using masculine or ambiguous pronouns despite his feminine physical characteristics. There is also Kite, a male mentor to Gon who is reincarnated into the body of a girl. After their reincarnation, Kite refers to themselves using both masculine and feminine pronouns, indicating that they are gender-fluid. While none of these characters are ever referred to with labels like transgender, the plot makes it clear that all of these characters identify with genders different than the ones they were born with.
While Marvel and DC have featured a handful of trans characters in the past few years, most of them are easily forgotten or ignored within the wide scope of each universe. Both companies have been attempting to get better at this, of course. DC recently incorporated Supergirl's Dreamer into the main DC universe. Marvel on the other hand has Squirrel Girl's friend Koi Boi, and Hulk ally Charlene McGowan. While all of these great characters are important steps forward, each is a periphery character who has yet to receive prominent comic stories focussed on them. Meanwhile, Hunter x Hunter gives room for its transgender characters to be powerful and important people who drive the stories they're in.