Hundred Points

A perfect test score, frozen into an emoji: the red "100" with the double underline copies the mark a teacher scrawls on flawless work, and it carries that meaning straight into chat — total agreement, absolute authenticity, "you nailed it". The African American English phrase "keep it 100" — keep it a hundred, meaning stay completely real — is bound up in the symbol so tightly that the emoji often stands in for the words.

As emphasis it works like a stamp of approval you drop after someone else's point, or double up for extra force. The underline is doing quiet work: it is what makes the number read as a graded mark rather than a plain quantity, and it is why the glyph looks broken if a font renders the digits without it. Worth a note that it means approval, not the number one hundred — using it where you actually mean a count reads as a small category error, the visual equivalent of writing "literally" when you mean "figuratively". When you want to say something is entirely, unarguably right, this is the single character that does it.