To obtain this achievement you need to get the ending that results in you romancing Kasumi. To do that you need to follow these choices exactly. Feel free to skip through the dialogue in between to speed things along.
-Only a bit
-Say nothing
-Disagree
-Fight back
-Stay in class
-Stay Silent or This is your fault (doesn't matter)
-Ask what happen to Hikaru
-No
-Challenge to a race
-Because I know I am better then you
-Go along with it
-Very close
-Break up with Arata
-Tell a lie
-Challenge Rika