The song was first uploaded on January 4, 2012 as one of the tracks featured in the compilation Kagamination. Unlike Hayakawa-P's Yuragi no Uchuu, Koi to Mira or Live-P's Saiaku no Carnival and several others which were uploaded the day the compilation was uploaded, iroha(sasaki) uploaded the song "sigh" half a year after the compilation was first distributed on VOC@LOID M@STER.
Like the booklet illustrated by Suzunosuke, the PV is also done by the same person. It's also said that the version uploaded to Niconico is remastered from the one used on the compilation.
"sigh" tells a story of Rin, who kept pondering on why she keeps on sighing. One day, an angel (Len) came and took away all her sighs away. Although she was happy then, eventually a bitter and painful childhood memory came to picture where Rin's "important person" passed away (Len).
She desperately tried to warm Len up with her "sighs" to no avail. The angel tells her that he's sorry he's so late, and that he wants her to be happy and not sigh anymore. After he leaves, Rin finally smiles.