One common storyline is that Penny's sibling might be on a quest to find out more about her past. They could be searching for the secrets behind her creation, especially if they were created in a similar way. Another is that they get involved in the battles at Beacon Academy, perhaps to protect Penny's legacy or continue her mission of protecting the people.
Often, Penny's sibling in fanfiction is shown to have unique abilities, just like her. They might use these abilities to team up with Team RWBY or other characters. For example, if they have some sort of technological powers, they could be helping to upgrade the weapons or defenses of their allies. They could also be a target for the bad guys, who might want to use their powers for evil.
Their very existence can drive the overarching plot forward. Maybe there are secrets about Penny's creation that her sibling holds the key to. This could lead to a whole new adventure where the characters have to protect the sibling from those who would do anything to get that information, like Salem's forces. It could also lead to a discovery that changes the whole understanding of how technology and humanity interact in the RWBY world.
One common theme is friendship. Despite their differences, many fanfictions explore how Mercury and Penny could form an unlikely friendship. Another theme is redemption. For Mercury, who has a rather complex and often villainous role, fanfictions may show him on a path of redemption with Penny's influence.
Well, the theme of acceptance is quite prevalent. The OC sibling may be different in some way, like having unique abilities or a different background, and they have to be accepted by Blake and the other RWBY characters. Also, the theme of sacrifice often comes up. The sibling might sacrifice their own desires or opportunities for the sake of Blake or the greater good in Remnant. In some fanfictions, this sacrifice can be very emotional and impactful.
One common theme is the exploration of Penny's unique existence. Since the SI is reincarnated as Penny, there's often a focus on her robotic nature and how it affects her perception of the world. Another theme can be friendship, as Penny was always eager to make friends in RWBY. The SI might have a different take on forging relationships within the RWBY universe. Also, there could be themes related to identity. The SI has to come to terms with being Penny, who has a pre - established identity in the show.
Ruby is often shown as being deeply affected, with feelings of guilt and a strong determination for revenge. Yang may become more protective of Ruby, fearing she'll get too involved in dangerous situations. And Weiss could be seen as more solemn, using Penny's death as motivation to fight harder for justice in the fanfiction.
One common theme is family. Since he's Penny's brother, stories often explore their family bond. Another could be identity, as he might struggle with who he is in the RWBY world.
Another is their career - related stories. For instance, Penny might be trying to break into acting in a more serious way and the guys help her in some unexpected ways. Howard could be working on a big project at NASA and the others are there for support. Raj, being an astrophysicist, might be involved in some international research and share his experiences with the group. And Leonard is constantly dealing with the pressures of his work at the university, and his friends are there to cheer him up or give him advice.
One common theme is friendship. Since they are both characters in the RWBY world, many fanfictions explore how they become friends and support each other. For example, Penny might help Ciel with her training, or Ciel might stand up for Penny when others misunderstand her.
In RWBY Penny and Ruby fanfic, common themes are aplenty. Friendship is a big one. They are depicted as having a really strong friendship. The idea of identity also pops up a lot. Penny's identity as an android and how she fits into the world with Ruby by her side. Additionally, protection is a theme sometimes. Ruby might feel the need to protect Penny because of her unique nature, and vice versa. There are also themes related to the exploration of the RWBY world together, like uncovering secrets and facing challenges as a team.
Well, this kind of fanfiction often explores unique concepts. It might involve the character from RWBY being reborn as Penny. Maybe it'll show how the character adapts to Penny's life, her abilities and relationships. It could also bring in new storylines that are different from the original RWBY plot.