Who is Sergei from Dreams of My Russian Summers and what is their importance?

Sergei is Charlotte's son born as the product of her gang rape in the desert. Even though Fyodor knows that Sergei is not his son, he still raises him and loves him as his own. Sergie looks darker than the rest of his family and is described as having a cool, even droning personality that doesn't change even in the most stressful situations.