Complex sentence: By chance, during a walk in a park Toru meets Kizuki's ex-girlfriend Naoko, and they grow close since they both share the same loss but as they grow even closer, Naoko's sense of loss also grows.
Split into simpler sentences: By chance, during a walk in a park Toru meets Kizuki's ex-girlfriend Naoko, and they grow close since they both share the same loss. As Toru and Naoko grow even closer, Naoko's sense of loss also grows.