That would be Chester Bloom!

Chester, Kendra, Amy, Nadia, Towerz, Chris ... they are now victims of what's known as the "Chuck Cunningham syndrome":

http://en.wikipedia.org/wiki/Chuck_Cunningham_syndrome

By their definition, Chuck Cunningham syndrome is "a term of criticism, applied when a regular character in a television series is removed with little or no explanation."

The six characters mentioned above most definitely apply!

P.S., if anybody remembers the exact seasons when those half dozen characters made their last appearances and never returned, by all means, make the submission to the wikipedia article!