In 2 Timothy 4:11, who does Paul say is the only one with him?

Answer: Luke

Demas had deserted Paul and others had gone elsewhere, so Paul writes, "Only Luke is with me. Get Mark and bring him with you."

“because Demas, in his love of this world, has deserted me and gone to Thessalonica. Crescens has gone to Galatia, and Titus to Dalmatia. Only Luke is with me. Get Mark and bring him with you, because he is useful to me in the ministry.”— 2 Timothy 4:10-11 (BSB)