Welcome to the timeless classic that has captivated minds and fingers for decades – Tetris! A puzzle game that challenges players to strategically rotate, move, and place falling shapes called Tetrominoes across a rectangular matrix, aiming to create and clear horizontal lines without gaps. The game's simple yet profound concept was conceived by Alexey Pajitnov in 1984, and it has since evolved into a cultural icon across various platforms.

**All Characters and Levels of the Game**

Traditionally, Tetris doesn't feature characters as it is a puzzle game focused on geometric shapes. There are seven unique Tetrominoes – I, O, T, S, Z, J, and L – each with its distinct form. As for levels, Tetris progresses in difficulty with increasing levels. Each level speeds up the fall of Tetrominoes, requiring quicker thinking and faster reflexes. Levels can range from the serene beginning to the near-impossible blitz at high numbers.

**What Attracts Users Most to the Game**

The allure of Tetris lies in its straightforward yet addictive gameplay. It provides an intellectual challenge coupled with the satisfaction of clearing lines, often accompanied by the thrill of narrowly escaping defeat as the pieces stack higher. Its accessibility and universal appeal transcend age and cultural barriers, making it a game that's instantly recognizable and widely enjoyable.

**Why It Is Popular**

Tetris has remained popular due to its infinite playability; no two games are ever the same. It's a game that exercises the brain, offering a mental workout that’s both fun and beneficial. Its simple design means it can be played on virtually any device, from graphic calculators to the latest smartphones. Furthermore, the nostalgic value and the iconic Tetris tune have contributed greatly to its enduring popularity.

**Release and Versions**

Originally released in 1984, Tetris has seen countless versions on a multitude of platforms. Over the years, there have been variations with added features, multiplayer capabilities, and different thematic elements, but all remain true to the core mechanics. As of my knowledge cutoff in 2023, there are numerous Tetris games available, with the latest including modern twists and graphics.

**The Most Difficult Part of the Game**

Arguably, the most challenging aspect of Tetris is the increase in speed as the player progresses through the levels. The quick decision-making required at high levels can be daunting, as well as dealing with the infamous 'S' and 'Z' Tetrominoes which often disrupt plans and lead to an increase in stack height.

**Strengths and Weaknesses of the Game**

Tetris's strengths are its simplicity, ease of understanding, and the immediate satisfaction it can provide. However, its weaknesses may include a lack of diverse gameplay modes and variations, which may deter players seeking more complex experiences with narratives or character development.

**Other Games by the Developer**

Alexey Pajitnov, the inventor of Tetris, has worked on several other puzzle games after his revolutionary creation. These include titles like "Welltris," which presented a 3D variant of Tetris, and "Hexic," a tile-matching game. It's important to note, however, that Tetris has been licensed and developed by a range of studios over the years, so not all versions are directly from Pajitnov's hands.