1. Introduction of the game:
Slither.io is an online multiplayer game that fuses elements of classic arcade games like Snake with the modern massively multiplayer online gaming experience. Players control an ever-growing worm (or a snake) with the objective of becoming the largest in the server. The game is played in a petri dish-like arena, where players must eat pellets to grow while avoiding collisions with other players, which leads to their demise.

2. All characters and levels of the game:
Slither.io doesn’t have distinct characters or levels in the traditional gaming sense. Instead, all players begin as small, identical worms, and their objective is to consume colored pellets from other defeated players and randomly spawned ones in the environment to grow in size and length. The level of the game is an endless, flat arena where the gameplay continues until the player's snake dies.

3. What attracts users most to the game:
Users are most attracted to slither.io's simple yet challenging gameplay. It’s easy to understand but difficult to master, creating an addictive loop of trying to beat one's own score or outlast other players. The game's competitive aspect, combined with the instant playability and the satisfaction of growing your worm, make it particularly engaging.

4. Why it is popular:
Slither.io became popular due to its casual game mechanics, the ability for quick gameplay sessions, and it being accessible on multiple platforms including web browsers and mobile devices. Its competitive nature and the fact that it doesn't require a player to log in makes it an easy pickup-and-play game for all ages. The game's simplicity coupled with a high skill ceiling makes it appealing for both casual and competitive players.

5. The most difficult part of the game:
The most difficult part of slither.io lies in surviving as your worm becomes larger. As a player grows, they become a bigger target for others who may be looking to cut them off and consume their remnants. Navigating the increasingly crowded space while avoiding a sudden demise is a continual challenge.

6. Strengths and weaknesses of the game:
The strengths of slither.io include its simple controls, cross-platform playability, and the fact that no registration is required to jump into the action. Additionally, its replayability and the social competitive aspects are strong points. However, its weaknesses could be the lack of depth in terms of gameplay variety, which might not hold the interest of players looking for a more robust gaming experience. Some players may also experience lag due to server issues caused by the high volume of players.

7. What other games have been developed by the same developer:
Slither.io was developed by Steve Howse. Prior to slither.io, he developed the game Flappy 2048 Extreme, which was a combination of Flappy Bird and 2048. Post-slither.io, Howse has remained focused on maintaining and updating slither.io and hasn't released new titles, opting instead to keep improving the game that made him well-known in the gaming community.