Diamondfin Koda is a Deluvian monitor who trained at the Diamond Reef monastery.
He was sent to accompany Rollo invisibly and protect him until he had delivered the Deluvian peace offer. As such, he technically didn't need to catch the arrow fired at him by one of the Vistrian troops on the ramparts, since he'd already delivered the message, but figured he may as well get in the opportunity to show off and threaten some surface-dwellers.
Interestingly, Deluvian high command ordered him not only not to attack anyone except in self-defence, but also specifically not to kill King Edward von Smith or King Hando van Smith. Given that Hando is an old man and both Kings were very far from the front line, the Deluvians must have anticipated Koda taking a very liberal interpretation of "self-defence". This insight into his personality is backed by his proclomation that, regarding the decision whether or not to accept the treaty, he "hope[s] you [the Vistrians] choose slaughter". Nice guy.
In an Extras, Demonac revealed that had one of the party decided to attack Koda, they would probably have won, but only after suffering heavy casualties among the Vistrian troops. This may mean that Koda was overconfident in his taunting, although it is also possible that he is so zealous that he is willing to give his life to keep the war going.