This is an
onomatopoeia for laughter. Its origin is the Korean
onomatopoeia ㅋㅋㅋ, in which ㅋ stands for the "k" sound, like in raspy, stifled laughter. (I'm not sure why it wound up transliterated as "kekeke" rather than just "kkk", though.) Incidentally, the same
onomatopoeia also exists in Japanese, written as けけけ.