Summer is the main antagonist of "Summer love" episode in Hey Arnold!. She showed some interest in his sandcastle building and thought he was a great artist. the two hang out, they did things like lay on a raft together in the ocean, eat lobster on the peer, and lied on beach towels in the sand.
However, it is revealed that she's actually seeing another boy Sandy when Helga eavesdrops on Summer's conversation on the pier, and that she is just using Arnold because she wanted to win the sandcastle competition. The prize of the contest being a guest appearance on the TV show "Babewatch".
The next day Arnold is still being deceived by her. They take some pictures together in the arcade after watching the sun come up. Then they build a sandcastle together, as she planned Sandy comes over and compliments Arnold on the sandcastle, Summer acts like she doesn't know the guy and plays along in the plan, she gets a yes out of Arnold, that he'll enter the competition with her.
On the day of the sandcastle competition, things go bad for Summer when Arnold is listening in on her and Sandy's conversation and discovers the truth thanks to Helga. Arnold then blows Summer off and tells her that he knows now that she was just using him the entire time. He leaves her shocked and stunned.
Summer enters the competition with Sandy instead, and they lose out to Arnold and Helga. Summer is in tears, Sandy tries to comfort her, but she just punches him in the stomach with her elbow and they break up while Arnold and Helga appear on "Babewatch."