The 'former' MI6 spy Christopher Steele wrote the dossier. The current CIA spy Eric Ciaramella wrote the 'complaint':

Federal documents reveal that the 33-year-old Ciaramella, a registered Democrat held over from the Obama White House, previously worked with former Vice President Joe Biden and former CIA Director John Brennan, a vocal critic of Trump who helped initiate the Russia "collusion" investigation of the Trump campaign during the 2016 election.
Further, Ciaramella (pronounced char-a-MEL-ah) left his National Security Council posting in the White House's West Wing in mid-2017 amid concerns about negative leaks to the media. He has since returned to CIA headquarters in Langley, Virginia.

"He was accused of working against Trump and leaking against Trump," said a former NSC official, speaking on condition of anonymity to discuss intelligence matters.

Also, Ciaramella huddled for "guidance" with the staff of House Intelligence Committee Chairman Adam Schiff, including former colleagues also held over from the Obama era whom Schiff's office had recently recruited from the NSC. Schiff is the lead prosecutor in the impeachment inquiry.