Detroit Metal City, Aggressive Retsuko Recommendations
Both feature seemingly mild-mannered main characters with death metal alter-egos (complete with hardcore forehead tattoo) getting to comedic situations and trying to cope with life.
The difference is Aggressive Retsuko's heavy metal side is internal and usually hidden while DMC's hardcore alter ego is literally an idol.
Both shows feature a seemingly harmless and docile character who has a death metal alter ego with a kanji on their foreheads. \m/