Lynne Marie Stewart, who starred in "Pee-wee's Playhouse" opposite Paul Reubens, has died at 78.

The actor's rep confirmed to USA Today that Stewart died after a tumor was found near her liver and gallbladder in mid-December. Prior to her diagnosis, she had been struggling with exhaustion.

"This was totally unexpected, and she passed a month or so after her diagnosis," Smith said.

"Lynne was the most wonderful actress. She was the kindest and (most) honorable person," Smith continued, adding that Stewart "loved the craft" and "didn't care" about receiving "pay or press."

Stewart rose to prominence for her role as Miss Yvonne on "Pee-wee's Playhouse" from 1986 to 1990, alongside Reubens, who died in July 2023 after a battle with cancer.

Stewart's passion for acting began in high school at Beverly Hills High and continued at LA City College where she studied theater arts and formed a close friendship with Cindy Williams of "Laverne & Shirley." She later honed her comedy skills at The Groundlings, the iconic LA sketch comedy troupe where she was an alum.

Since 2005, Stewart has played Bonnie Kelly, the mother of Charlie Day's character, on the hit FX sitcom "It's Always Sunny in Philadelphia." She also appeared in multiple "Pee-wee" projects, the classic 1973 film "American Graffiti," 2003's "Tracey Ullman in the Trailer Tales," and 2011's "Bridesmaids."

Additional film credits also include "The Running Man" (1987), "Clear and Present Danger" (1994), and "Pirates of Silicon Valley" (1999). Her representative confirmed that she recently finished filming "The Dink," a new pickleball movie starring Ben Stiller.

Since news of her death, dozens of friends and colleagues have taken to social media to mourn. Leading the tributes was Stewart's friend, "Elvira" actor Cassandra Peterson.

"My heart is breaking upon hearing the news of my dear friend @lynnestewart78 Lynne Stewart's passing," Peterson wrote in an Instagram post. "One of the kindest, sweetest, funniest women who ever lived. The iconic Miss Yvonne of Pee-wee's Playhouse: She'll always be 'the most beautiful woman in Puppetland.'"