He becomes the boss of Lynn Sr. when the latter quits his previous job to train as a chef at the Aloha Comrade. He is forced to lay off Lynn after receiving "the rudest phone call of his life" from Lola when the Loud children find out their father is a dishwasher at the restaurant, unaware that their father had changed jobs.
Left without help during the breakfast rush he initially refuses to hire Lynn back, until the children all vouch for him and admit to their mistake. When Sergei sprains his cooking arm on a pile of filthy dishes during this he nearly closes shop until Lynn Sr. and his children all volunteer to fill in. Enjoying a huge spike in business due to Lynn's cooking, Sergei immediately promotes Lynn to co-chef.
Judging from his accent, it can be assumed that Sergei is originally from Russia.