Mackie is a former member of Löded Diper who plays the band's electrical guitar. He has a little sister and is presumably in college, studying robotics. He serves as major character in Diper Överlöde and will appear in the animated adaption of Rodrick Rules. It's unknown how he ended up joining the band.
Löded Diper guitarist (formerly)
College student (presumably)
Diary of a Wimpy Kid: Diper Överlöde
Diary of a Wimpy Kid: Rodrick Rules (2022 film)