WARNING: The following contains spoilers for Episode 1 of The Slime Diaries "The Residents of the City of Monsters" now streaming on Crunchyroll.
The Slime Diaries is the newest spin-off of That Time I Got Reincarnated As A Slime, focusing on the daily lives of the characters and the menial tasks that bring joy to each one of them. It takes place sometime between Season 1 and Season 2, while the citizens of Tempest settle into their new life, building their city and living freely. The first episode sets this up pretty well and doesn't forget to reinforce a fact that the original anime consistently brings up: that Shion is best girl.
Shion was an ogre before she met Rimuru and ever since the destruction of her village, she has been by his side as his number one secretary. She's notoriously clumsy and sometimes even dense, but she makes it up with her consistent hard work and loyal dedication to Rimuru and the life they're trying to build for all monsters. This combination of obliviousness and fierce caring is what makes Shion the best girl of the original anime.
The first episode of The Slime Diaries brings up this idea almost immediately, using a mundane, daily activity to further show how great Shion is. Shion comes into Rimuru's office to cut up some fresh paper, saying that "For Great Rimuru, as always, I will give all I've got!" and proceeds to aggressively cut a giant block of wood into many sheets of paper, drowning Rimuru in the load.
Afterward, she offers Rimuru some tea, and, in her excitement, spills it all over him. Rimuru responds by pointing out the frequency of her mistakes and the quality of the tea itself, comparing it to poison. He says that she's the perfect secretary, as long as she doesn't move, as it's all in her looks. Instead of taking this as the insult it so obviously is, Shion only hears the praise that's veiled beneath the disdain.
This proves that, to Shion, as long as Rimuru is getting something out of her hard work, she's succeeding. Her loyalty to Rimuru knows no bounds, so even when she makes egregious mistakes, it's just endearing. This is what makes her best girl -- even when she falters, Shion always finds the positives and uses them to keep pushing forward and work even harder. Her sometimes annoying persistence is what keeps her around and keeps people engaged in her character. And, like Rimuru mentions, it doesn't help that her physical appearance is so bodacious.
The Slime Diaries, while different in tone and content from That Time I Got Reincarnated As a Slime, keeps to the same messages of the original anime, especially in regards to who the characters are and the role they play. Shion is Rimuru's number one secretary and is privy to all important matters. Even though Rimuru often treats Shion with obvious annoyance, it's clear that he cares about her very much. In Season 2 of That Time I Got Reincarnated As A Slime, Rimuru's main motivation even stems from Shion and her possible loss. Shion really is best girl, to both fans and Rimuru, and the new spin-off makes sure to acknowledge this.