It would likely make Draco more self - reflective. Discovering a twin would force him to question his own identity. He might start to see himself not just as a Malfoy heir but as an individual separate from the expectations placed on him by his family.
One possible plot twist could be that the secret twin has been living in the Muggle world all this time. When Draco discovers his twin, he has to navigate the challenges of introducing a Muggle - raised sibling into the wizarding world. This could cause a stir in the Malfoy family, as they have strong pure - blood beliefs.
Well, in the fanfiction, they could have a very close relationship. Maybe they protect each other from the dark forces at Hogwarts. They might share secrets that no one else knows, like about their family's hidden past or their own personal fears and dreams.
She could be really kind. In contrast to Draco's sometimes haughty attitude, she might be the one who is always helping out the first - years, showing them around Hogwarts and making them feel welcome.
The twin could change a lot. He might be the one who stops Draco from making some really bad decisions. For instance, if Draco was about to follow Voldemort's orders blindly, the twin could convince him otherwise. This would then change the whole course of Draco's actions and how he interacts with Harry Potter and his friends.
Well, they could have a very complex relationship. On one hand, they might be extremely competitive, always trying to outdo each other in everything from academics to impressing their parents. Since they are in Slytherin, they could also team up and be a formidable duo, using their combined wits to get what they want in the wizarding world. Maybe they have a secret pact to protect each other no matter what.
One plot idea could be that Draco's twin sister is secretly a supporter of the Order of the Phoenix. She could be working undercover within the Slytherin house to pass information to the good side. Another idea is that she has a special magical ability that Draco doesn't, like the power to communicate with magical creatures in a unique way. This could lead to adventures where she uses this ability to solve problems or help her friends. Also, she might have a love story with a Gryffindor, which would cause a lot of tension between her and her family.
Maybe the story is about their rivalry. Sol is jealous of Draco's popularity among their Slytherin peers. He tries to outdo Draco in everything, from academics to Quidditch. This causes a rift between them. However, when Draco gets seriously injured in a Quidditch match, Sol realizes how much he actually cares about his brother. He then helps Draco recover and they reconcile, vowing to support each other instead of competing all the time.
At first, she may avoid Harry Potter altogether. However, if she discovers something that links her family's past to Harry in a way that she didn't expect, it could draw her closer to him. For example, if there was a hidden connection between the Malfoy family and Harry's parents. As she delves deeper into this connection, she might start to protect Harry from other Slytherins or even from her own brother at times. Their relationship could be one of mutual discovery and slowly - growing trust as they face the threats in the wizarding world together.