Hermione's intelligence is a big factor. She can come up with clever plans and solutions that the twins might not think of on their own. This helps them in their pranks and their business endeavors at Weasley's Wizard Wheezes. For instance, she might use her knowledge of ancient spells to create new and exciting products for the shop.
Hermione has a passion for learning and discovery, which matches the twins' enthusiasm for inventing and exploring new magic. They can bounce ideas off each other. She can provide the theoretical knowledge while they bring the practical and creative aspects. Her ability to adapt to different situations also makes her a great mate. Whether it's dealing with a difficult Hogwarts professor or a tricky magical situation, Hermione can handle it, and this quality is valuable when she's with Fred and George in the fanfiction world.
Her loyalty is another aspect. Once she considers them as her mates, she will stand by Fred and George through thick and thin. In the dangerous wizarding world, this loyalty is crucial. She would fight alongside them if they were ever in trouble, just like she did for Harry. Also, her strong moral compass can keep the twins in check when their pranks might go a bit too far. She can make sure they don't cross any ethical lines while still having fun.