There are at least 5 artists with this name:

1) Skatepunk one-man band from Chile (current artist image).
2) Metal band from the USA.
3) Dutch Power Electronics, side project of Terg and Cultus.
4) Hardcore band from Helsinki, Finland
5) Four piece Latinx queer band from Atlanta who formed in 2016.

1) Formerly a regular 4-piece skatepunk band born in Chile back in 2004, Bitter has become the solo project of Ricardo Araya since late 2008. Playing all the instruments but drums, he has managed to stay melodic and technical, now with the help of Impacted Records (USA). http://www.myspace.com/bitterskatepunk

2) Bitter is a popular metal band in the USA signed to No Explanation Records.
www.noexplanationrecords.com/epk/bitter/

Known for their powerful live performances the band released their 2nd album Headcase worldwide on No Explanation Records 11/2009
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Artist biography from last.fm