Ensign Rainer was a Starfleet tactical officer serving aboard the USS Enterprise-D in 2370.

He was assigned to the night watch and was on duty while Doctor Beverly Crusher substituted for Lieutenant Commander Data, who was retrieving crashed probe fragments from the planet Barkon IV. Rainer attempted to hail Data's shuttlecraft and inform him of a delay in the planned rendezvous with the Enterprise, but was unable to contact him. (TNG: "Thine Own Self")