Arai first appears in Ep 15 - The Refreshing Battle in Karuizawa!, as a middle-school friend of Haruhi's. He's a young man working a summer job in the resort town of Karuizawa as a delivery boy for his uncle's produce store. Through flashbacks and interactions with the hosts, it is seen that Arai is an intelligent, industrious, thoughtful and tolerant individual.
Not much is known about Arai, not even his first name. When he and Haruhi are in middle school together, he harbors a secret crush on her and even tries to ask Haruhi out, but she misinterprets his comments and unwittingly rejects. Haruhi and he meet again in when Arai delivers fresh groceries to Pension Misuzu. Seeing one another after over a year has passed, the former classmates enjoy chatting about their middle school days. The Hosts gather around to hear about Haruhi's pre-Ouran life and Arai admits that he used to crush on her, leading Hikaru to develop a dislike towards him.
While Haruhi and Hikaru are on a date together in Ep 16 - Operation Haruhi and Hikaru’s First Date! the clouds darken and Haruhi becomes worried about thunder. Running into Arai in town, the boy offers them a lift back to Pension Misuzu, but Hikaru snaps and tears off, with Haruhi chasing after. Arai contacts Misuzu a bit later to learn that neither Haruhi nor Hikaru has yet gotten back, which is when Tamaki goes off the deep end upon discovering that Hikaru abandoned Haruhi in the storm, commanding him to locate Haruhi immediately because of her brontophobia. Hikaru learns a big lesson about thinking about the feelings of others that day and when Arai shows up the following morning with a fresh watermelon to share, Hikaru genuinely makes amends.