The Kindaichi Case Files/Heartwarming

Return Series

• The friendship between Riku Busujima and Marina Togami.
• The message that Marina left with a recorder given by Riku before she died. It was not for her parents, sister Erina, or lover Reiji Aoi, but in fact for Riku, a guy she only knew and befriended with for a short time. Knowing that she may not survive, Marina chose to leave the message as an undeniable proof that Riku was not involved with her tormentors Tamaki and Uozaki, rather than allowing his life to be destroyed by them, proving that although Marina didn't return Riku his feeling, she still loved and cared for him in her own way. 
• The anime added a scene with Riku shedding tears after he heard that Marina still believing him even beyond death.

Meta

• It is implied that death penalty generally discouraged in in the world of Kindaichi (at least in the manga & anime version, the drama version still implies that it's still in use per real life Japanese laws), as Sympathetic Murderer such as Marina Ayatsuji had committed more than three first degree murders yet received at most a Life Sentence (Takato Yoichi, who was sentenced to death in 37 Series, is an obvious exception since he piled crime after crime upon himself), but Kenmochi also adds that there is still a chance for them to receive parole, providing they served sufficient amount of time, shown remorse for their actions, and improved themselves for the better during their imprisonment. Added with that many received emotional support from Hajime and the friends/loved ones, and had regained a will to live, it's not far to say that they'll lead a better life with their second chance after being released.
  • A bit from above, Kindaichi 37 years old is set 20 years after many of the cases, so many of them (that didn't commit too many crimes) should've been released.