Castillo is one of the top players on the market ahead of Tuesday's trade deadline. The Reds are last in the NL Central, and the talented right-hander could help push a contender over the top.
"It's been an absolute pleasure to be around Luis," Reds manager David Bell said. "He is a Cincinnati Red. That's how I see it. I absolutely love him and so does everyone else in the clubhouse. But we know it's a business and none of us will stay together forever.''
Castillo (4-4) allowed three runs -- including JJ Bleday's first big league homer -- and six hits. He also struck out eight and walked one in his first start since July 14.
"He's the talk of this trade deadline and you see why,'' Marlins manager Don Mattingly said.
Castillo gave Cincinnati fans one last moment after Jesus Sanchez's RBI double for Miami left runners on second and third with none out in the seventh.
He responded with three straight strikeouts -- all swinging. He walked off the mound to a standing ovation, took off his cap, and gestured to the crowd.
"Everything turned out as well as it could have,'' Castillo said through an interpreter. "That last inning was marvelous, two runners on and I was able to punch out the last three. Really it is up to the people upstairs now what happens."
Fortes led off the third with his fourth homer, and Bleday opened the fourth with a drive to right in his fifth big league game.
The 29-year-old Castillo signed a one-year, $7.35 million contract in March, avoiding arbitration. He missed all of spring training due to a right shoulder strain and didn't make his season debut until May 9. But he is rolling now, improving to 2-0 with a 1.58 ERA in his past five starts.
"I don't know what is going to happen next," Castillo said. "I have been here for five years. The Reds are the team that gave me the opportunity to pitch in the big leagues. I would like to be in a Reds uniform the rest of my life, but baseball is a business.''
The Associated Press contributed to this report.