Well, El Cid has a basis in history. Rodrigo D铆az de Vivar was a real person, but the stories about him have likely incorporated fictional elements over time to make them more captivating.
Their relationship can be developed through shared adventures. For example, if they go on a quest together to retrieve a powerful artifact. During this journey, they have to rely on each other's skills. Cid's combat and strategic skills combined with Rose's knowledge of the local area or certain magic can bring them closer. They start to respect each other more, which is the first step in developing their relationship.
I'm not sure specifically what 'cid kevi fanfic' is about as it could be a very specific or niche fanfic. It might be related to characters named Cid and Kevi, perhaps their adventures, relationships, or some unique storylines created by a fan.
In a CID fanfic of a playful love story, the characters could be in a situation where they are forced to work together closely. Maybe they are undercover at a party. They have to pretend to be a couple, and during this time, their pretend relationship starts to feel real. Their playful interactions like sharing secret glances and inside jokes make the story really engaging.