She is Sian's long-time childhood friend and the object of his unrequited love.
They have lived next door to each other since they were five years old in kindergarten.
Unable to bring himself to confess, he let time slip away, and now she has a boyfriend.

Listening to stories about her trips with her boyfriend, hearing about their fights, even helping her pick out gifts for him…
That is how he stays by her side, under the name of a 'friend.'

Sian sends her a message.
[What are you doing this weekend?]

He waits for a reply, lost in his thoughts.

Ah… is she meeting her boyfriend..?
I miss her.