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It's been a while, hasn't it? In what kind of world do we have to twist and turn to become who we want to be? Do we have to trust ourselves? Can we trust ourselves? Was my stuffed teddy bear's name actually 'Bear Bear' or was he a figment of my imagination? Can I trust myself on that? Sometimes I wonder where Bear Bear went. I never saw him after I moved from my childhood home. Was he even real? Can he hear me now? Would he be proud?
— Author's comment
"ECHO" is an original song by CIRCRUSH, a collaboration between Crusher-P and Circus-P. It became very popular shortly after its upload on Niconico Video, reaching #1 on Weekly VOCALOID Ranking #367 and receiving many derivative works. "ECHO" has entered the Hall of Legend (July 9, 2015), reached over twenty two million YouTube views, is featured as a bonus track in Warner Japan's compilation album Download feat.初音ミク, and has been featured on TBS's ranking show "Rank Oukoku" as 2nd most popular VOCALOID song of 2014 on Niconico Video. "ECHO" is also available in JOYSOUND and Club DAM, allowing it to be sung in Japanese karaoke bars.
The lyrics to "ECHO" are vague, and can be interpreted in many different ways; Crusher-P has expressly stated that there is no particular meaning to ECHO's lyrics, and that some fans have explained the lyrics better than Crusher-P could herself.
The name of the character in the video is "Primadonna".
On October 8, 2015, the one year anniversary of the song, it was announced that ECHO would be receiving a Japanese novel adaption. It will be written by Akira and illustrated by Oguchi, one of the official illustrators for Kantai Collection. A promotional video was uploaded on February 12, 2016.