Digging through the archives, I came across this page (https://dhscomix.com/comics/comic.php?v=39&p=Filler1) from eight years ago - of course, in hindsight, Mort and Colfer are obvious, but who's the scaly?
That's Mason!
He first appeared a very very long time ago in chapter 12 (https://dhscomix.com/comics/comic.php?v=12&p=17), and got a minor intro in the new kids volume 47 (https://dhscomix.com/comics/comic.php?v=47&p=9)! He doesn't get up to much but hangs around with Mort sometimes.