"I never imagined you saw me as a woman, Senpai..."
After leaving Sekai's house, Setsuna recalls her encounter with Makoto earlier that day. They reminisced about when they first met at the entrance ceremony, and Setsuna mentions that she has a job near the beach. Makoto introduces her to his little sister, Itaru, and suggests that Setsuna should come hang out after work, as he's usually in the area of the beach during the day.
The next day at work, Setsuna begins having difficulties after encountering some unruly customers, including Katsura Manami. After leaving work, she stops to look at the ocean. Makoto sneaks up on her and asks her what she's up to. Sensing that something is bothering her, he asks if her boyfriend dumped her, to which Setsuna replies she didn't have one to begin with. She then asks Makoto if he has a girlfriend, to which he lies and says yes. After some prodding for info, Karen walks in on the conversation, and Makoto says that Karen is his girlfriend. Slightly taken aback, Karen plays along with the lie. Dejectedly, Setsuna goes home.
The chapter then skips ahead to later in the summer, where Itaru and Karen are playing on the beach. Karen approaches Makoto, who is looking wistfully at the boardwalk. When asked what he's looking for, Makoto says "nothing, really." Karen jokingly accused him of looking for cute girls, and that he's not allowed to do that, because he's her boyfriend.
It's implied that Karen and Makoto actually began dating after the scene with Setsuna (possibly with some reluctance on Makoto's behalf), and that Makoto and Setuna do not encounter each other again for the remainder of the summer.
Getting Sweet Summer for chapter 2 involves a specific path through the discussion with Setsuna the day of her first encounter with Manami.
In general, you need to get the conversation on the topic of whether Makoto has a girlfriend or not. This means that when Makoto sneaks up on Setsuna, he should ask her if "she's doing something." Setsuna deflects, and Makoto asks "what's wrong?" Setsuna deflects again, but Makoto can tell that's not the entire truth, asking "are you worried about something?".Setsuna continues to deflect, but by now Makoto has determined that she's upset.
Makoto then guesses that she was "dumped by her boyfriend." Setsuna will say that she didn't have one, and proceed to ask Makoto whether he has a girlfriend. If he says "I do," Setsuna will begin prodding him with questions about her, trying to find out who she is.
As this is entirely a lie, and Sweet Summer ends with Karen being Makoto's supposed girlfriend, answers to these questions must not logically preclude Karen from being Makoto's girlfriend. To achieve this, Makoto says that "she doesn't go to our school" and then either claims that she's "younger than me" or declines to answer further.
Eventually, Karen will walk in on the conversation, and Setsuna will ask if she is Makoto's girlfriend. When Makoto answers "she is" (or "yeah"), the chapter will end with Sweet Summer.
The ending scene of Sweet Summer is very similar to Summer As Usual, with Karen taking the place of Otome.