In Case 2, "Lumpy Ice Princess," you will come across Sunny, the character created as part of a contest they were running during the development of the game. It is a blonde, blue-skinned inhabitant in the back road when you're performing the Three Noble Deeds section of the case. As such, it'd be rather hard to miss seeing this character, though it is possible you might not talk to him. However, all you have to do is just initiate a conversation to get this achievement.
You can find Sunny again in Case 4, "Everything Must Go!!" during the second part or in Case 5, "Album (Re)Cover", part one. So if you miss him the first time, chat up with him again when you get here.