1. Introduction of the game:
"7 Deadly Sins: Grand Cross" is an enthralling mobile RPG game that merges cinematic storytelling with tactical combat and card-based gameplay. Developed and published by Netmarble, this game plunges players into the beloved world of the anime series "The Seven Deadly Sins," allowing them to experience the adventures of its vibrant cast in an interactive format.

2. All characters and levels of the game:
The game features an extensive roster of characters drawn directly from the anime, each with unique abilities, attributes, and backstories. These characters are categorized into levels of rarity and power – ranging from the common R-grade to the highly sought after SSR-grade heroes. As players progress through the game's narrative and battles, they can unlock new characters and enhance their abilities through leveling up, awakening, and equipping powerful gear.

3. What attracts users most to the game:
Users are most enticed by the game's rich graphics and animations that faithfully represent the anime's aesthetic. The strategic depth afforded by the card-based combat system, combined with team synergy and the timing of skill use, adds a layer of tactical gameplay that is rewarding and engaging. Additionally, the inclusion of voice-acted cutscenes and the original series’ storyline weaves in a compelling narrative aspect that pulls fans of the series and newcomers alike into the game's world.

4. Why it is popular:
"7 Deadly Sins: Grand Cross" is popular because it offers a comprehensive package of intriguing gameplay, stunning visuals, and an expansive story that resonates with fans of the anime and RPG enthusiasts in general. The game's regularly updated content, including collaborative events and new challenges, also contributes to its sustained popularity and replayability.

5. When the game was released and how many versions are currently available:
The global version of "7 Deadly Sins: Grand Cross" was released in March 2020, following a successful launch in Japan in 2019. Since its release, numerous updates and seasonal events have been introduced, bringing fresh content and features to the ongoing game. While there is technically one base game, the frequent updates can be viewed as additional versions that evolve the game over time.

6. The most difficult part of the game:
One of the most challenging aspects of "7 Deadly Sins: Grand Cross" is the PvP (Player vs Player) arena where players battle against each other's carefully assembled teams in real-time. Success in this mode requires a deep understanding of character synergies, gear optimization, and strategic planning. Moreover, end-game content like the Demon Raids and Death Matches demand high-level coordination and preparation, testing the skill and dedication of the most seasoned players.

7. Strengths and weaknesses of the game:
The game's strengths lie in its high-quality animation, engaging storyline, and strategic gameplay, which are enhanced with regular content updates. However, like many mobile RPGs, it faces criticism for its in-game monetization tactics that can lead to a "pay-to-win" environment for more competitive aspects. Some players may also find the resource grind for character progression to be time-consuming and demanding.

8. What other games have been developed by the same developer:
Netmarble, the developer behind "7 Deadly Sins: Grand Cross," is a well-known mobile game publisher responsible for other popular titles such as "Lineage 2: Revolution," "MARVEL Future Fight," "BTS WORLD," and "King of Fighters All-Star." Each game showcases Netmarble's expertise in creating immersive mobile gaming experiences across various genres and IPs.