Well, often there's a theme of self - discovery. The puppet, which might be seen as a simple creation at first, could discover its true purpose or abilities with the help of BB. There can also be a theme of redemption. Maybe the puppet has made some mistakes in the past and BB helps it to make amends. And a theme of cooperation is common too. They have to work together to achieve a common goal, like saving their world from a great danger.
To create compelling characters, writers should give them distinct personalities. For the puppet, it could be naive yet curious. For BB, perhaps it's brave and a bit mischievous. Also, give them clear goals. The puppet might want to find its place in the world, and BB could be aiming to protect something important.
One common theme could be friendship or an unlikely alliance. Given the nature of the characters in the FNAF universe, they might be forced to work together against a common threat.
One common theme could be mystery. Given the nature of the characters, there might be elements of an unknown backstory or a hidden connection between them that the fanfic explores. Another theme could be redemption, perhaps the purple guy trying to make up for his actions in relation to the puppet.
One idea could be a story where Mangle and Puppet team up to solve a mystery in the haunted pizzeria. They could discover some long - forgotten secrets about the place and form a strong bond during their adventure.
One common theme is friendship. Despite their spooky nature, many fanfics explore a deep and meaningful friendship between the puppet and Golden Freddy. Another theme is mystery. Their origins and the secrets of the world they're in are often a big part of the story.
In a plot, the Puppet might be trying to bring peace to the haunted pizzeria. Golden Freddy, initially reluctant, is eventually convinced by the Puppet's determination. They face many spooky and dangerous situations together, like dealing with other malevolent animatronics. As they work side by side, they start to care for each other and the pizzeria slowly becomes a less terrifying place.
Another idea is that they have a past connection that is slowly revealed throughout the fanfic. For example, they might have been created by the same person long ago and share a unique bond because of it. This could lead to a story full of flashbacks and emotional revelations.
I'm not sure about specific 'security puppet x lefty' fanfic. You can try searching on some popular fanfic platforms like Archive of Our Own. They usually have a wide range of fanfictions on various topics.
I don't have the information about who created 'bb chan fanfic'. It could be a fan with a great deal of creativity and passion for the character bb chan.