Kate and Castle are the main protagonists. Kate, as she endures the experience of giving birth to what is described as a 'big baby'. Castle, on the other hand, is there with her, being an important part of the story as he shares in this significant moment. There could potentially be other minor characters like family members who might come to visit later, but the focus is on Kate and Castle.
Perhaps in the fanfiction, Kate's pregnancy was a big surprise to everyone. When she's giving birth to this 'big baby', Castle is by her side, reminiscing about their journey together. The plot could involve the medical team being astonished by the size of the baby and how they handle the situation. It might also show how Kate and Castle prepare for the new addition to their family in the days following the birth.
The main characters are likely Castle and Kate. Castle is the one who believes Kate is alive, so he's a central figure in this fanfic. Kate, whether she's present or just the subject of Castle's belief, is also a main character as the story revolves around her being alive.
The Cullen family usually plays a significant role. Carlisle, with his medical knowledge (even if it's vampire - related in the Twilight context), might be involved in ensuring a safe birth. Alice could be there, perhaps using her gift to foresee any potential problems during the birth. Even the other members of the Cullen family would be around to offer their support in different ways.
Pikachu's Pokémon friends could also be main characters. For example, Pikachu's best friend, a Squirtle, could be there to offer moral support. It could bring little gifts like small shiny stones to cheer Pikachu up during the pregnancy and be there when the baby is born.
It could be a horror movie or the Alien series.
Obviously, Shikadai's parents would be central characters. His mother would be going through the process of pregnancy and childbirth, and his father would be there for support. Also, the medical staff or midwives in their world would be involved, taking care of the mother and baby during the birth.
Bella's mother or a close female friend could also be main characters. Her mother could share her own experiences of giving birth and offer practical advice. The close female friend could be there to hold Bella's hand, offer comfort, and maybe even crack a few jokes to lighten the mood during the intense moments of the birth.
As it's a Gacha Life story, the main character is likely the big baby. But there could be other important characters too. Maybe there's a wise old character who guides the big baby, or a mischievous sidekick that the big baby teams up with. It all depends on how the creator of the story designed it.
There are many possible fanfictions on this topic. One could imagine that in the fanfiction, Penny's pregnancy might bring a new dynamic to her relationship with Leonard. For example, they could become more protective of each other and start planning for their future as parents more seriously. It might also affect their friendships with Sheldon and the others. Sheldon, who is not always good with changes, might initially struggle to adapt to Penny being pregnant and the new baby in their lives.
The baby, of course. Growing up without a mother would be a huge part of the story. The father would also be deeply affected, having to raise the child alone and deal with the grief of losing his partner.