Mr. R. Zelner, introduced in season 5, is Rachel's boss at Ralph Lauren. Rachel inadvertently kisses him during her job interview but still gets hired. He seems very timid and serious about his job at Ralph Lauren.
He also has a son, Ross, who is interested in dinosaurs, which Ross (Geller) uses as an avenue to convince him to offer Rachel her job back when she's fired after being caught interviewing for a job with Gucci. He also thinks that Ross's (Geller) name is "Ron."
It would appear that Mr. Zelner is very paranoid about his job. In "The One Where Rachel Goes Back To Work", he says that he would follow Ralph Lauren "into hell" before whispering "Microphones..." to Gavin Mitchell.