At the moment, it seems the two current routes we have are:
Spoiler Warning
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1. Free Twig, she's introduced to Thea after you return from the spiral city, and slowly learns that she isn't a slave any longer. You and Thea both view Twig as a 'little sister', and by the end of the current content for that route, there's no romance and from the way MC has spoken to Twig, I'm unsure if there will be any romance content for that route in the future. Potentially.
2. Slave Twig, once she's secured in the basement she has 4 stages essentially, and I believe the 'love' subroute is by treating her well as a sex slave. By Stage 3/4, during sex, Twig will thank the MC and then say that she loves them. Currently the content ends at Stage 4, 10 obedience, and hopefully in future content we'll be able to take Slave Twig out of the basement and have some way to explain to Thea.
Slave Twig also has a 'harsh' subroute, like the 'love' subroute, if you consistently treat Twig harshly throughout the 4 stages.