Unlike previous Descendents bassists Tony Lombardo and Doug Carrion, who both used a pick, Alvarez plays finger style bass, and he also provides backing vocals when live (and lead vocals as heard in "Cause" on the All Live Plus One album).

In the summer of 2006 he joined Gypsy Punk band Gogol Bordello for part of the Van's Warped Tour and the Reading and Leeds Festivals.

Alvarez currently plays guitar and sings in Endless Monster and the Vultures.

[citation needed] In 2006 Alvarez played on The Lemonheads self-titled comeback album which was released on Los Angeles' Vagrant Records.

[citation needed] Australian punk band Frenzal Rhomb wrote a song about Karl, on their 2011 album, Smoko At The Pet Food Factory, called "Alvarez".