The X-Men alum, who dated Michelle Trachtenberg from 2004 to 2006, shared a tribute to the late actress following news of her death Feb. 26 at the age of 39.

“Incredibly sad to hear about Michelle’s passing,” Shawn, 45, wrote on Instagram, alongside a red carpet photo of the pair. “She was an incredible person and I will always remember the years we spent together fondly. She was loving, quirky and would never pass up a law and order SVU marathon.”

He also acknowledged her family, adding, “My condolences to her mother Lana and sister Irene! This is an incredible loss!”

Michelle and Shawn dated in the early aughts after she used her humor and confidence to woo him at a 2004 Oscars party.

“Well, I put out an ad in the classifieds: ‘Wanted, superhero. I’m a damsel in distress,’” she quipped in a 2005 interview with BlackFilm.com. “No, we just met. He hates anything publicity [and] we met at an Oscar party and he was just like Mr. Grumpy unhappy to be out in public and I totally approached him.”

The Inspector Gadget star continued, “I was the forward one. I take full credit.”

However, in more recent years, Michelle had been going through difficult times.

“She struggled the last few years,” Michelle’s Harriet the Spy costar Rosie O’Donnell said in a Feb. 26 statement to E!. “I wish I could have helped.”

Calling her death “heartbreaking,” Rosie added, “I loved her very much.”

New York police responded to a 911 call around 8 a.m. on Feb. 26 and found Michelle unresponsive in her apartment. The actress—who reportedly underwent a liver transplant recently—was pronounced dead by EMS at the scene. NYPD officials confirmed to E! News that her death is not being investigated as suspicious and her cause of death will be determined following an autopsy.

Since the Gossip Girl alum’s passing, several other stars have expressed their sadness over the untimely tragedy.

Kenan Thompson also shared a touching tribute to his fellow Nickelodeon alum alongside a throwback video of the pair at the 1999 Nickelodeon’s Kids Choice Awards.

“Our first Nick movie star has departed us!!” the Saturday Night Live comedian wrote in the Feb. 26 Instagram post. “She was my friend and now she rests!! Check on your people!!!”

Meanwhile, Gossip Girl showrunners Josh Schwartz and Stephanie Savage, who worked with Michelle during her time on the CW series, reflected on her talent, which ultimately helped expand her role.

“We are deeply saddened by the news of Michelle’s passing,” the writers shared in a statement to NBC News. “Michelle was so funny, kind, and talented. Her portrayal of Georgina Sparks was an iconic fan favorite, and grew from a one season villain to a beloved character who returned over the course of six seasons.”

They added, “She was a delight to have on set and will be deeply missed. Our thoughts are with her family.”