Ray Singh is Susie's first crush, first kiss, and first love. He and Susie were just getting to know each other when she died. The love letter he slipped into her schoolbooks on the day of her death is only read by her from heaven. Seeing Susie taken from her life right at the moment she's embarking on her first romance makes her death seem all the more cruel. Susie doesn't dwell on Ray's process of grief, but it's obvious that this early tragic loss marks his life.
Other than his kissing experiments with Ruth, it doesn't seem that Ray had any romantic relationships after Susie's death. It looks like he was a virgin too when Susie borrows Ruth's body and makes love to him. Ray's romantic encounter with Susie/Ruth probably seems to have the effect of freeing all involved from being bound unhealthily together. Afterwards, Ruth and Ray can feel free to live again, and Susie to move on up to the wide wide Heaven.