i think it supposed to be 아 진짜 너ㅋ (ah its really you k(something like okay*) or maybe 아 진짜 너 ㅋㅋㅋㅋㅋㅋ (ah its really you hahaha or kekekeke) maybe due to fast typing so that 너ㅋ(ㅋㅋㅋ) turns into 넠 usually korean syllable is composed of 2- 3 characters depending on how it supposed to pronounce. ex.: 아 (ah) this is composed of 2 characters the "ㅇ" (ng) but it doesn't have phonetic sound it is place before the vowel and "ㅏ" (a) and "kimchibokkeumbap" is right coz i do it sometimes 넠ㅋ to look cute ㅋㅋㅋㅋㅋㅋㅋ ^ㅇ^

Yes, you are correct. Their laughing sound "ㅋㅋㅋㅋㅋㅋㅋㅋ" overlapped as a suffix to 너 due to just fast typing or to look cute.