Koiwai is a 32-year old man, he is slim, medium height and has rather pale looking skin with more narrow looking eyes, his hair being short and parted on the left side, both his hair and eyes being of a light bluish purple color.
He is often seen in his causal business look which consists of a dress shirt with a red tie - the sleeves being rolled up to his elbows paired with blue trousers and black dress shoes.
Koiwai has a rather energetic and easygoing personality.
He has a very mocking humor sometimes a bit sarcastic, as said by Yuuta Sakurai, everyone around him enjoys his presence for his constant jokes; he specially likes to make fun of Sakurai since he is always too serious and a bit dramatic about his problems.
Koiwai on the other side cares a lot about his friends, in the past he was the only person who seem to ever sit up and listen to Moriko during a phone conversation in wich he realized how tired she was because of her hard work. The same attitude is observed with Sakurai, Koiwai always listen to his problems and even pulls the strings to help him.
On the other side is clear that Koiwai acts like a womanizer, he is a really smooth talker as he flirts with Moriko Morioka since the first time they speak and never stops to do it during the whole anime, however, its notorious that he is not really much interested in her, and his honey words are but a mean to tease Moriko's lack of self confidence since even before they meet he was already aware of the feelings of his friend.
Koiwai often interferes between Yuuta Sakurai and Moriko Morioka by pulling the strings to make things happen, as he seems to be far more experienced in human relationships than both of them, he usually manipulate events to put them together, or simply make them speak.
During a good part of the story he intentionally causes jealousy to Sakurai in order to get him some reaction, Koiwai seems to be really sure that his friends will end together, for example: Even if Sakurai drops the triple date and says he wont contact Moriko, he goes on with his plan and gets Moriko to realize Yuuta is attracted to her.
- Koiwai's blood type is O.
- Koiwai's star sign is Gemini.
- Koiwai is 32 years old, which makes him the second oldest character shown, only behind Kazuomi Fujimoto's unnamed coworker (Anime info only).
- During his date with Moriko Morioka, Koiwai confirmed that even if he is not adept to them, he indeed like and plays video games.
- On various occasions, Koiwai is seen to have overtime in his job where he did in the prior company and so he does in his actual job.