Roxana Castillo was an Oceanic Airlines ticket agent who approached by Sun at Heathrow Airport. As Sun asked for a ticket to Los Angeles and handed over her passport, Roxana told her to wait a moment. Sun was taken to a interrogation room where she was locked up, and approached by Charles Widmore, who claimed that "they" (presumably the airport staff or the Oceanic Airlines employees) acted on his instructions.
Although the character was credited as ticket agent only, her name can be seen on her badge.
The name Roxana Castillo also appeared on the guest list for the benefit concert in the flash sideways world, but it is unlikely to have any canoncial connection with Roxana the ticket agent, as Roxana Castillo is just a common name used on props.
The prop of Roxanne's flight ticket also had her full name listed as Roxana Castillo.