Kero is the Japanese onomatopoeia for a frog's croak, and it became a recognisable meme sound through Tsuyu Asui, the frog-like hero known as Froppy in My Hero Academia, whose sentences end with a flat 'kero.' The published note for this clip points to Froppy as its inspiration, and at just 235 milliseconds it works as a bite-size reaction rather than a quote. Short anime verbal tics travel well on soundboards, much like the famous tuturu greeting, because one syllable can carry a whole character. That is why this tiny ribbit keeps resurfacing under frog videos and cute anime edits.