If you're looking for anime similar to Demon Slayer: Kimetsu no Yaiba - The Flower of Happiness (Light Novel), you might like these titles.
Tanjiro and his friends accompany the Hashira Tengen Uzui to an entertainment district where Tengen’s female ninja agents were gathering information on a demon before they suddenly disappeared. In order to investigate, Tanjiro and the others disguise themselves as women to sneak in!
For the first time in over 100 years, the upper ranks of Muzan Kibutsuji’s Twelve Kizuki are not full. Angered, Muzan dispatches the survivors on another mission. Elsewhere, Tanjiro journeys to a village of swordsmiths and has to explain how his sword was so badly damaged to Haganezuka, the smith who made it. While Tanjiro waits for his sword to be repaired, enemies close in…
Tanjirou and the group have completed their rehabilitation training at the Butterfly Mansion, and they arrive to their next mission on the Mugen Train, where over 40 people have disappeared in a very short period of time. Tanjirou and Nezuko, along with Zenitsu and Inosuke, join one of the most powerful swordsmen within the Demon Slayer Corps, Flame Hashira Rengoku Kyoujurou, to face the demon aboard the Mugen Train on track to despair.
Youko Tama and her younger step brother, Jinka Sendou, are two demon siblings traveling through sengoku-era Japan to keep the peace and battle evil. Joined by Hyoudou Shinsuke, a cowardly swordsman, the people they meet, places they see, and creatures they battle will be legendary!
Rin Okumura is a seemingly ordinary boy living at the Southern Cross Boys' Monastery. One day, however, Rin encounters a boy who has been possessed by a demon. Amidst the struggle, he not only awakens latent powers sealed within him, but also learns that he is the son of Satan! No longer able to stay under the protection of the church and with his very existence straddling the line between human and demon, Rin decides to fight against his parentage and joins the True Cross Order as an exorcist. Now, the teen must attend the True Cross Academy and begin special exorcist training alongside his new comrades all while keeping his identity a secret; but can the son of Satan really become a true exorcist?
Ken Asuka was prepared to die, but even that privilege was taken from him. In the aftermath of the Great War, Japan was a confusing place, one that didn't seem to have a place for a suicide pilot like Ken, his plan having broken down before he could complete his mission. Frustrated over losing his way in life, struggling with feelings of guilt that his friends died yet he alone carried on living, Ken knew no peace. However, after a run-in with some American soldiers, Ken rediscovered feelings he'd carried since his youth. A practitioner of martial arts form a young age, he had abandoned that path for the war. But now, it might just be his ticket to a new purpose in life! Though not everyone will seek to use his power for good.