In the beginning of Season Two, Arya helps Jaqen to escape. Jaqen says something like "The God of Death have been cheated of 3 lives. The girl must name 3 names" etc etc etc. During the coming episodes, Arya could name a person to die and Jaqen would kill him, within minutes. In S2E10, we see Jaqen take the name Valar Morghulis, and more importantly, change his face. Also, he implies everyone in Braavos knows him.
This made me and my girlfriend think: Who/what is Jaqen? Changing one's face isn't normal, even in a world where magic (although is seems to be quite rare) exists. What we (my GF and me) agreed on was, is that Jaqen is an avatar of the God of Death, or maybe an angel (or something similar) of the God of Death.
Can anyone help shed some light on this?