Sumiko Kobayashi/Relationships

Ninzaburo Shiratori
As a child, Sumiko instantly became Shiratori's true love, and was the very reason why he entered the police force. They had confronted a bunch of shoplifters together as children; Shiratori had prevented her from getting hurt and also stopped the theft, and as thanks she gave him a cup of cola with a band made of straw wrappers, shaped like the sakura blossoms that look like the symbol of the Japanese Police.

For quite a while Shiratori believed that Miwako Sato was the girl he had loved since childhood and changed his life forever, due to her resemblance with the "girl of destiny", hence he was interested in her. At the beginning of a case that revealed parts of Shiratori's past, he realized Sato wasn't his true love, and even if she was, she had chosen Wataru Takagi over him. Even worse, at the end of the said case, Shiratori was left depressed and exhausted, since another woman who could have been his long-lost love actually turned out to be the culprit. However, Ayumi cheered him up by saying the exact same lines that the girl from his past said and by making the same design on the wristband she gave him. Shiratori desperately asked who taught her that, and Ayumi told him it was her school teacher, Sumiko. Then, at that exact moment, Sumiko came to pick up the Detective Boys upon Juzo Megure's request.

When Shiratori laid his eyes on Sumiko, he instantly recognized her as his true "woman of destiny", and their relationship began to spark. He would always warn her of any dangers nearby where she worked or lived, which caused Sumiko to develop romantic feelings. In fact, his concern over her, constant visits, and kindness towards her, often caused her to blush deeply.

When Sumiko became a witness in a murder investigation, she learned of her resemblance to Detective Sato. And after hearing about Shiratori's former crush on her from other policemen, she thought that the only reason he was so interested in her was because of her striking resemblance to Sato. This left her not only shocked, but also hurt, thinking Shiratori didn't have feelings for her. This wasn't helped by Shiratori not having told her about their shared past, as he was waiting for the right moment to do so.

Not knowing the real reason why Shiratori liked her, Sumiko began to act coldly and harshly towards him. Even when he confessed his true feelings to her and told her about their meeting, she brushed it off, thinking Shiratori confused her with Detective Sato. However, it was all cleared up by the end of the case, when he not only saved her from the culprit, but also coolly recited a law the same way he did as a child in that book store, which finally reminded her of their first meeting. Shiratori romantically confessed to her, which made her blush. She replied with a quick and simple "Yes".

Afterwards, the two began dating, and Sumiko started giving Shiratori bentos every morning for his "energy demanding" work. On White Day, Shiratori was seen with many presents inside his car for her. Later on, she was shown with all the delicious sweets Shiratori had brought for her, at her house, happily not knowing where to begin.