The Emo is a pun on the multi-class Elementalist Monk which is shown as "E/Mo".
The E/Mo is a flag running build for GvG. The traditional E/Mo uses Air or Water Magic along with Healing or Protection Prayers to run the flag, defend against splits, or offer defensive support at the flag stand. It is rarely used to solo NPCs or other flaggers, however it can be used to assist teammates do so.
There is also the Emo Bonder, which got popular thanks to UWSC. This describes an elementalist who maintains Ether Renewal with various enchanments so as to maintain bonds such as Protective Bond on multiple targets.