Review Detail of Ur_Mom_Gay in The Last Rising Sun (Assassin)

Mc's decided not to show his strength because he sensed that some people are watching him, but how the fuck did he get so powerful after graduation? it felt as if he is been training since academy time. I felt that it made no sense when Mc decided to hide his strength without explanation at first, but after he mentioned that he was being watched it felt like another plot hole are being opened, as I mentioned, how did he get so powerful? in my opinion, The author's attempts at justifying Mc's behaviour during the academy arc just resulted in another plot hole