Do you mean in the romance and relational aspect?
In the first book, there are two primary sources of conflict. Alicia's first emotional conflict is trying to force a recovery after the anxiety and depression she falls into due to the car accident. But their relational struggle comes from a number of aspects: 1. Kattar's refusing to express his feelings for Alicia and choosing to lie, and deny it (even at the risk of her feelings) instead. 2. The fact that this struggle kept her from being adopted by his mother, and yet he still didn't ask Alicia out, and more.
I create tension in a lot of ways, but mostly with the way that I utilize silence. Refusing to answer questions. Moments where you are left to question what someone is thinking because they won't say it.
And romantic tension in a number of ways, though my two favorite instances are the scene in "White Hot Sugar" where Alicia notices for the first time that Kattar has gotten really bad at hiding his feelings for her, and the scene in "Burgundy" where he wants to comfort her, but hesitates to do so, because he's in love with her, but she already has a boyfriend.
I think the key for creating romantic tension is along the same lines as emotional tension, things left unsaid, where you just hint toward thoughts and feelings.