Rob Nelson, Head of Development and Co-Director at Rockstar North, discussed the role Jason and Lucia's relationship will play in GTA 6 in an interview with IGN. He stated that the romantic storyline will be an important part of the narrative, but players who aren't interested don't need to actively develop the relationship between the two protagonists.
"If you don't want to do it, don't. It's up to you," Nelson explained. Throughout the story, various situations will constantly force Lucia and Jason to stay together and act as a team, meaning they'll spend most of the game as partners regardless. Meanwhile, Rockstar doesn't want to force romance on players. The developers hope that interested players can feel the emotional connection between the characters, while others can focus on the criminal adventure.
For players who want to spend more time together, GTA 6 features a variety of shared activities. Lucia and Jason can go for walks, hold hands, visit the zoo, go diving, skydiving, stroll along the beach, kayak, and, of course, pull off heists together. According to Nelson, being able to play as both protagonists should create a sense that players care about their relationship and see them as a unit.