John Kosciuszko was born in the United States to a Polish father and American mother. John grew up fascinated with history and language, which he focused on as he continued his studies at university. While he originally held dual citizenship, he ultimately gave up his Polish citizenship when joining the US Military, as he felt a strong sense of duty and honor in serving his country, and his polyglot nature would prove to be beneficial to this end. After his term in the military was up John somehow found himself in Eastern Europe, though his specific role and affiliation in the area is not currently known.
It is unknown if John arrived in Chernarus before or after the outbreak (and if this is related to his prior line of work), considering the preceding events, but it is clear that John has no intention of sticking around the area long term ultimately planning to head a long distance West. He has been attempting to keep a low profile in the area in order to not draw attention to himself among any larger groups or significant people in Chernarus.
John once considered himself to be a "good" person (depending on the interpretation), even when the current situation in the world proves this to be incredibly difficult. However, the struggles he faced have led to the times testing his conscience, and now John can not help but focus on himself when necessary. While he may not always make good choices, he tries to make what he believes is the right choice for whatever instance he finds himself in. He is loyal to the few he places his trust in and can at times be kind and empathetic to others, but understands that now that society as a whole has collapsed, he has to place himself before others when it is a matter of survival in the post-apocalyptic world.