Michelle Trachtenberg was a wonderful person who died much too young ... so says her "Buffy the Vampire Slayer" costar.

We spoke with James Marsters -- a veteran actor who played punk vampire Spike in 97 episodes of the hit TV show -- and, he says his heart is heavy today because of the loss of Michelle.

Marsters says the world has "lost a beautiful soul" and remembers Michelle as being "fiercely intelligent, howlingly funny, and a very talented person."

He adds she passed away far too young ... and left behind many people who knew and loved her. Marsters says his heart goes out to her family -- and, he asks people to give them the space to heal during this incredibly difficult time.

He finished off his comments to us with a message meant for Trachtenberg ... "Godspeed Michelle. You are missed."

As we told you ... law enforcement sources told us police and paramedics responded to a call this morning for a woman in cardiac arrest and found Trachtenberg unresponsive. They pronounced her dead at the scene.

Other sources told us she received a liver transplant in the last year ... unclear if that played any part in her death.

Michelle appeared on 66 episodes of 'Buffy' as Dawn Summers, the younger sister of Sarah Michelle Gellar's titular character. She and James often appeared on screen together.

James is one of several costars to pay tribute to Michelle ... with her "Gossip Girl" pal Ed Westwick sending out prayers after he heard the news. Michelle was 39.