What happens when two childhood friends see two people kissing?

From Shoujo Sense:

The packed morning train isn't a good place to be if you're small; a fact that high schooler An knows all too well. So when a boy rescues her from the crowd one morning, she should be grateful... right? The only problem is, he treats her like a little kid! Who is her rescuer, and what will happen when she meets him again...?