His only connection to her might just be that of a shadow. Her shadow is very common, even if Souna is thrown into the crowd, it won't be that special, but she and her shadow are very similar, a moment of meeting, a moment of love, a moment of separation, he loves that shadow, not another shadow that is the same as her...
If you love someone, you have to look at his or her shadow, because the shadow cannot tell the expression or heart, nor can you tell who you really love.