We dont know yet if he is human (could have become a mixed specie like Cain) and from my interpretation he recognised his eyes by them having that ininteressed look, his brother would not have had the same vibe since they were polar opposite. Hence why the former companions (and traitors) didn't recognise him.
At least that is how I read it but next chapters will probably delve deeper into this subject (づ￣ ³￣)づ
I don't think its based by how the MC would be recognized by human or not, nor by his brother's ex companion or not. Yeon-woo (MC) and Jeong-woo (YOUNGER BROTHER) are identical twins but are completely different from each other. So based on how the ex-companion remembered Jeong-woo, they remembered as the kind, soft-spoken, and humble person whereas Yeon-woo is the ruthless, straight-forward and cruel person. That is why they don't recognize the MC as the one they know because their vibes are completely different. The reason why the human one know was because he knew him from their military days compared to the ex-companion who had absolutely no idea that the one they know had a twin.
But he recognized Cain himself, not his brother.
The others have never seen Cain. They have different personalities, so the others definitely won't be able to connect them just based on the eyes.
It’s because he knows Cain personally, so he recognized him by his eyes. The others only know his brother and even if they’re twins Cain is a cold blooded killer while his brother wasn’t, his eyes were probably a lot softer and not full of pure murderous intent.
this creates plot holes, a human person able to recognize him based on the eyes and yet ex- companions of his dead brother didn't recognized him????
lazy writing