Kenan Thompson Mourns 'All That' Costar Christy Knowings

Kenan Thompson is mourning his fellow "All That" star Christy Knowings ... saying her death is unimaginable.

Kenan paid tribute to Christy on social media Thursday, shortly after we first reported her death, writing ... "Never could have imagined this day!!"

The 'SNL' star called Christy "a real one" and "one of the funniest people out here" ... while sending love to her family and adding, "Gonna miss you buddy!!!"

Kenan and Christy appeared together on Nickelodeon's "All That" in the late 1990s ... Christy joined the sketch-comedy series in 1997 and remained through Season 6.

As we first reported ... a family member told us Christy was hospitalized Friday after suffering an asthma attack that left her with brain damage.

She remained on life support until Tuesday, when her family made the difficult decision to take her off it. Christy died that night at a Los Angeles-area hospital.