Jan is a character in episode 13 credited as being voiced by famous voice actress Rie Kugimiya.
In episode 14 a girl refers to Jan by name at 16m50s, telling him to quit drinking and come dance. It is pronounced "Yan".
He and Thomas are mentioned 5m10s into episode 16 as being wanted to go to England and Hans and Emma were chosen by Dorothea in addition to them.
In episode 16 Polly is seen talking to him about a job she wants to apply for in England. He reads the employment details.
Polly tells Jan they might need a footman too, so that is his job.
Both dreams are dashed later when Hans explains that it is a scam.
Blonde-haired Jan is later seen accompanying the brown-haired Thomas, both wearing top hats, as they run off to tour England in episode 17.