Jean-Luc Picard
In a parallel universe, Jean-Luc Picard was a Starfleet officer and the captain of the USS Enterprise during the 24th century.
He sent Commander Riker on a mission to check up on mining operations on Naia VII. After an accident there, he beamed down to the planet himself. He told Geordi La Forge that Starfleet was cutting corners in safety to rebuild the fleet after their losses to the Borg at Wolf 359.
After Riker, Data and Dr. Crusher met the Eleventh Doctor, Amy Pond and Rory Williams in the holodeck of the Enterprise, Picard met them in the ship's observation lounge. He was politely sceptical of the three visitors. Their discussion was interrupted by news of the attack on Delta IV. He recognised the Borg's ships around the planet and agreed with the Doctor's sentiment that they "shouldn't be here". (COMIC: Assimilation²)
Other references
Whilst under the influence of the Syntax, the Eighth Doctor's companion Izzy Sinclair hallucinated about Captain Picard. (PROSE: Syntax)
Behind the scenes
The brief appearance of Captain Picard in Syntax treats him as a fictional character (as do, implicitly, other references to Star Trek and Star Trek: The Next Generation in Doctor Who stories). Assimilation², of course, treats him as a "real" individual. Reconciling these diegetic levels is an exercise best left to the reader.
