The most significant character arcs in "Summer Time Rendering" include Ushio Amamiya's journey to uncover the truth behind his cousin's death, Mio Amamiya's transformation as she copes with past trauma, Kaito Toba's development in his relationship with Ushio and his mysterious connection to Hitogashima, and the complex evolution of relationships among the characters as secrets are revealed and bonds are tested. These character arcs drive the emotional core of the story and add depth to the narrative of "Summer Time Rendering."