Shizuka- Ryouka and Shunji
Testu- Nanase and Shounji
Restu- Haruka and Kouichi
Hitoshi (Nimura)- Haruka and Doctor Nimura
Married couples- Ryouka and Shounji, Nanase and Kouichi, Haruka and Dr. Nimura
Dead people- Shounji, Kouichi, and Nanase (same car accident)
Shounji and Kouichi are brothers, Shounji is younger.
They do not tell you Dr. Nimuras' first name and Hitoshi is always called Nimura. Shounji is also spelled Shunji in the manga.
Ryouka adopted Testu and Restu and raised them as twins since they are very much like their fathers.
Kouichi continued to cheat on Nanase, with the reasoning as they were soul mates, with Haruka even while married but stopped when Nanase became pregnant.
Haruka and Nanase fought over Kouichi, but Haruka lost due to the fact that Nanase became pregnant and lied to her by telling her it was Kouichi's, when it was actually Shounji's.
Shounji liked Nanase even though she was his brothers wife, but once Ryouka forcefully points out all of their flaws and confesses to Shounji, Shounji later develops great feelings for her over Nanase.
Haruka gave Testu away because she didn't want to keep looking back at her painful past. she also didn't abort him because she was hoping that Kouichi might come back to her some day. Little did she know, that when Shounji, Kouichi, and Nanase came back from their road trip, Nanase and Kouichi were going to divorce and Ryouka was going to tell Shounji she was pregnant.
Haruka had Nimura out of loyalty to carry out the line and met her husband through her parents choice.