Game Pigeon is an innovative and entertaining collection of multiplayer games designed exclusively for the iMessage app on iOS devices. Embedded within Apple's messaging service, Game Pigeon offers the ability to engage in playful competition with friends and family directly through text conversations, presenting a seamless integration of social connectivity and gaming.

**Characters and Levels of the Game**

Each game within Game Pigeon brings its unique set of characters and levels, ranging from the diverse pool tables of "8-Ball" to the strategic grids of "Battleship" and the wordy challenges of "Anagrams." While some games rely on classic simplicity with fewer characters or levels, such as "Checkers," others like "Cup Pong" may include various challenges and modes that add depth to their gameplay.

**What Attracts Users Most to the Game**

The versatility and the ease of access are among the top attractions of Game Pigeon. Users are particularly drawn to the convenience of playing multiple games without ever leaving the chat interface and the opportunity to provoke friendly rivalry in ongoing message threads. The diversity of games ensures that there is something for every type of player, whether they enjoy strategy, word puzzles, sports, or arcade classics.

**Why It Is Popular**

Game Pigeon's popularity is rooted in its accessibility and social integration. It capitalizes on the existing social connections established through iMessage, allowing users to quickly challenge contacts to a game. The elimination of additional downloads or separate app requirements coupled with its free-to-play model makes it an inviting choice for casual gaming among iOS users.

**Release and Versions**

Game Pigeon was first released on September 13, 2016, coinciding with the update of iOS 10, which introduced the capability for iMessage apps. As of the current knowledge cutoff date in 2023, there have been multiple updates and versions released, improving existing games and occasionally adding new ones to keep the platform fresh and engaging.

**The Most Difficult Part of the Game**

Considering Game Pigeon hosts a variety of games, the difficulty can vary significantly among them. Some players may find strategic games like "Chess" to be challenging, while others may struggle with the precision and timing required in "Darts." The most difficult part often depends on the individual player's skills and preferences.

**Strengths and Weaknesses**

Strengths of Game Pigeon include its integration with iMessage, the broad selection of games, the ease of use, and the social aspect of playing with friends. However, weaknesses might be noted in the limitations set by being bound to iOS, potentially excluding players on other platforms, and the reliance on friend participation for most games, which may not appeal to solo players.

**Other Games Developed by the Same Developer**

Game Pigeon is developed by Vitalii Zlotskii and his company Vitalii Zlotskii which, as of my knowledge cutoff, is known primarily for the Game Pigeon suite. As such, the developer may not have a widely diverse portfolio outside of these iMessage-based games, but it's always a good idea to check the latest app store listings or the developer’s website for any new offerings.