Solistein

Captain Solistein was an aide to Darth Vader at the time Obo Rin was writing the Catalog of Intelligent Life in the Galaxy under Vader's patronage.

When Major Herrit, an Imperial Intelligence officer, disputed Obo Rin's expense reports and credentials in a message to Vader, Solistein relayed Vader's reply. Through Solistein, Vader ordered that Rin be reimbursed and allowed to continue his work. Vader and Solistein did not address Herrit's criticism of Rin's qualifications in detail, though Solistein did claim that Vader had visited Sab Rufo (a planet where Rin claimed to have been educated, and which Herrit believed did not exist.)

Vader also had Solistein tell Herrit to personally obtain a Duinuogwuin corpse for Rin to dissect, a mission which Herrit did not survive.

Behind the scenes
Captain Solistein's branch of service is not specified. However, the tone and content of Solistein's reply to Major Herrit makes it seem as though Solistein outranked Herrit, and was therefore an Imperial Starfleet officer (since naval captains outrank majors.)