City of Thieves: How did Kolya help Lev develop into a young man?

Kolya is Lev’s best friend, though by the novel’s conclusion they had known each other for less than two weeks, and had been thrust together by circumstance. Kolya seems to live each moment to the fullest to Lev, even to a fault. Lev begins their relationship viewing Kolya as arrogant and reckless, but eventually comes to respect his confidence as a vital asset in their adventures, and he becomes Lev’s mentor and a figure similar to an older brother, educating him on romance and fighting. Kolya is described as handsome and confident - quite the foil to Lev, who is middling in his attractiveness and a self-described coward. He not only fights before Lev, he also lays with Sonya while Lev is trying to sleep in an adjacent room, further presenting the dichotomy between the two.