4Kato is pulling the move "exerting dominance"; if one has siblings, was part of a sports team, or was in a rough neighborhood/bathroom, then you know and quite likely pulled this move yourselves. Hell, even the show "It's Always Sunny In Philadelphia" depicted this move in its censored glory.
I was a bit too distracted by his unusually hung dick to piece what happened that killed the guy in the tactical armor. He used his exposed finger bones and somehow guess the gap in his opponent's invisible armor to pierce his neck?
It's a stylized version of [ 忍 ], which is the "Nin" used in "Ninja". It can be used to mean "conceal", "sneak", "hide", etc.
NIN, the organization, is spelled in English.