Hiroyuki and Akari have been best friends since they were children.  Akari will never forget how Hiroyuki helped her collect her fallen school books when they were young, and has probably been in love with him ever since.  Hiroyuki is a real ladies man, however, and every girl who meets him seems to fall under his spell, even Akari's best friend. 

The cel above is from the last few moments of the series, after Akari has been sick for a few days, Hiroyuki rushes to meet her to escort her to the school Christmas party as he realizes how much he has missed her walking to school with him everyday.  Akari then presents him with the sweater she has knitted him for Christmas and I think then Hiroyuki comes to an understanding of his feelings as well. :)