1. Introduction of the game:
"Obey Me!" is a popular role-playing game that combines elements of strategy, romance, and storytelling in a richly animated and interactive world. Developed by NTT Solmare Corporation, the game invites players into a fantastical setting where they navigate through the celestial bureaucracy and drama of the Devildom, attending the "RAD" (Royal Academy of Diavolo) as an exchange student.

2. All characters and levels of the game:
Players interact with a cast of seven demon brothers, each with unique personalities and backstories: Lucifer, the prideful but responsible leader; Mammon, the greedy yet lovable second-born; Leviathan, the envious otaku; Satan, the avatar of wrath with a love for books; Asmodeus, the embodiment of lust with a passion for beauty; Beelzebub, the glutton with a big heart; Belphegor, the slothful but cunning youngest brother. As players progress, they encounter various levels of personal challenges and story arcs, enhancing their relationship with each character through interactive storylines and tasks.

3. What attracts users most to the game:
"Obey Me!" captivates users with its engaging, character-driven narratives and the unique 'intimacy level' feature that changes the dynamics of the player's relationship with each demon brother based on their choices. The anime-style visuals, voice acting, and a diverse array of events and side stories keep players consistently engaged.

4. Why it is popular:
The game's popularity stems from its immersive storytelling, customizable interactions, and the compelling romance angle that allows players to form a deep connection with characters. Accessibility through mobile devices and its freemium model also play a key role in its widespread appeal.

5. When the game was released and how many versions are currently available:
"Obey Me!" was released on December 12, 2019. The game has undergone several updates and events since its release, enhancing the player experience and expanding content, but remains as a single evolving version rather than multiple distinct versions.

6. The most difficult part of the game:
The most challenging aspect of the game can vary per player, but many find achieving the preferred outcome in certain high-stakes scenarios or maximizing intimacy levels with the characters particularly tough, requiring strategic planning and time investment.

7. Strengths and weaknesses of the game:
Strengths include its captivating storytelling, quality art design, and a strong sense of player agency given through choices. However, some may find the energy mechanic, which limits gameplay unless using paid items, and the need for a lot of grinding to progress without spending real money, to be weaknesses.

8. What other games have been developed by the same developer:
NTT Solmare Corporation has developed numerous other games, notably the "Shall We Date?" series, which features a variety of dating simulation games like "Shall We Date?: Wizardess Heart", "Shall We Date?: Blood in Roses", and "Shall We Date?: Ninja Love", among others, each with its unique setting and characters.