When Fullmetal Alchemist was released in 2001 it was a breath of fresh air amidst a sea of manga all trying to replicate Dragon Ball's success, and a decade and a half later, Demon Slayer followed in its footsteps. Though there are more than a few superficial commonalities, the similarities run much deeper than initial appearances suggest.
Created by Hiromu Arakawa, Fullmetal Alchemist tells the tale of brothers Edward and Alphonse Elric. After an attempt at using alchemy to bring their mother back goes awry, Edward loses his arm and leg while Alphonse loses his entire body. Though Ed is able to bind his brother's soul to a suit of armor, the guilt he carries propels him to seek out the mythical Philosopher's Stone in order to restore his brother back to normal. On the flip side, Koyoharu Gotouge's Demon Slayer began publication in 2016 and follows siblings Tanjiro and Nezuko Kamado. After their entire family is slaughtered by a demon, Nezuko is transformed into a demon herself. Tanjiro then joins the Demon Slayer Corps in an attempt to find a cure for his sister.
Immediately the points of comparison are obvious. Both manga focus on an older brother's attempt to fix something that's gone horribly wrong with their younger sibling. Unlike the protagonists of typical shonen manga such as Naruto or One Piece who frequently have lofty goals related to following their dreams, Edward and Tanjiro have concrete motivations related to helping someone they care about. More than just a motivation though, their respective goals ripple out to shape their plots. Almost every plot point in Fullmetal Alchemist and Demon Slayer is the direct result of the protagonists taking steps toward curing their sibling. As a result of this singular focus, both series remove a great deal of glut and filler which famously plagues Naruto and Bleach. It's telling that Fullmetal Alchemist ran for only 27 volumes and Demon Slayer 24 whereas Naruto ran for 72, Bleach 74, while One Piece is still going with a staggering 101 volumes released at the time of writing.
More than just similarly motivated protagonists and pacing though, both manga are also relatively unafraid to blur the lines between hero and villain. Many of the 'heroes' that Ed and Al meet on their journey have done absolutely monstrous things, and many of the villains they face are lashing out against oppressive systems. Meanwhile, Demon Slayer complicates its own premise by introducing ruthless Demon Slayers more bloodthirsty than the demons they want to slay. Additionally, readers are invited to feel a great deal of sympathy for many of the demons Tanjiro slays. Much like Fullmetal Alchemist villains such as Scar, these antagonists are often victims of circumstance who had villainy thrust on them. As if making the comparison even more solid, both series also take place in a very similar time period, with modernization vs. tradition playing a significant thematic role.
Despite prevailing cultural knowledge in the west, manga is a diverse medium and shonen a diverse genre. Though long-running action-heavy series like Naruto, Dragon Ball, and One Piece may have made the genre globally famous, shonen manga can be a vehicle for a wide variety of series. Hiromu Arakawa completely redefined what was possible in a shonen manga with Fullmetal Alchemist by focusing on complex characters with concrete motivations, and by all appearances, Koyoharu Gotouge kept that spirit alive with the modern masterpiece that is Demon Slayer.