It could be a slow - burning relationship. Initially, Tywin might see the marriage as just another political move. But as he spends more time with the Stark oc, he discovers her unique qualities. Maybe she has a great love for music or art, which softens Tywin's cold heart a bit. The Stark oc, on the other hand, might initially be afraid of Tywin but then realizes that he is not all bad. Their relationship could then grow into a deep and complex love, with each of them changing for the better because of the other.
At first, it could be very tense. Tywin is used to being in control and having things his way, and the Stark oc would likely have her own ideas and values. So, they might start off butting heads. But as time goes on, they could start to understand each other better. The Stark oc might see that Tywin has his reasons for his actions, and Tywin could come to respect the Stark oc's strength and integrity.
The plot might focus on cultural clashes. The Lannisters and the Starks have different values and ways of life. In this fanfiction, it could show how the Stark oc (original character) tries to adapt to the Lannister way of life in Casterly Rock or how Tywin has to deal with the more 'northern' Stark traditions that his new wife adheres to. For example, the Stark oc might be more in tune with nature and the old gods, which could be a contrast to the Lannister's more materialistic and power - hungry ways.
In OC fanfiction involving Tywin Lannister and the Starks, Tywin's strategic mind is a central aspect of his portrayal. He is always looking for ways to strengthen his house's position. The Starks, on the other hand, are known for their integrity. An OC could be a healer, for instance, who treats the wounded from both sides during a battle. This OC's presence might lead to some unexpected interactions between Tywin and the Starks, like Tywin seeing the Starks in a more sympathetic light due to the OC's influence.
Often, there's a plotline where the OC is a servant or a low - born character who rises to prominence and has to deal with the politics of Tywin Lannister and the Starks. They might be caught in a web of lies and intrigue as they try to serve both sides without getting killed. Also, a plotline could involve an OC as a mage or a seer who foresees the destruction that the feud between the two houses will bring and tries to stop it.
In the show, their relationship was mostly one of power and manipulation. But in fanfiction, it could be more of a mentorship. Tywin might see Sansa as a protégé, teaching her how to manage estates and deal with political rivals. Sansa, in turn, could use this knowledge to her advantage later. For example, she could learn how to outwit Littlefinger with the skills Tywin taught her.
Their relationship could develop in a very complex way. At first, Arya would be full of rage towards Tywin. However, if she were to be captured by the Lannisters and Tywin were to show her a different side of himself, perhaps a more paternal side (even if it's just a ploy), Arya might start to question her hatred. On the other hand, Tywin might see Arya as a potential asset, someone he could mold in his own image. This could lead to a relationship that is part mentor - mentee and part captor - captive, with a lot of power dynamics and emotional turmoil.
At first, their interaction would likely be very guarded. Sansa would be cautious as she has learned to be in a world full of danger. Tywin would probably see her as a young, naive girl but also a valuable asset. As time goes on, Sansa might start to challenge Tywin's ideas, which could lead to some intense exchanges between them.
In such fanfiction, the relationship can be complex. Tywin Lannister is a complex character. An OC (Original Character) might be drawn to his power and authority at first. But his cold and calculating nature could also create conflict. For example, the OC might try to humanize him, which would be a difficult task considering his reputation.
Their relationship might start off very rocky. Given the history between their families, there would be a lot of mistrust. But as they spend more time together, they could start to see each other's good qualities. For instance, Ned Stark might notice the Lannister's intelligence, and the Lannister could come to admire Ned's integrity.
Often, Tywin is in a position of power over Sansa. So, he may interact with her in a condescending or manipulative way. For instance, he might use his authority to force her into certain decisions.
To develop the character, consider her upbringing. Growing up in the Lannister household would surely have an impact. She might have been trained in combat or in the art of negotiation. Her relationships with her family members are crucial too. Does she idolize her father? Or is she in constant conflict with him? Her values, such as loyalty and honor, can be shaped by her family's reputation but also by her own experiences. For example, if she witnesses an unjust act by the Lannisters, it could make her question their ways. Also, her goals in life, whether it's to gain power, find love, or something else entirely, will define her character further.