From The Heretic Knowledge Vault
A type of mage, first mentioned here, specializing in things martial, and employed by the Tsuiraku military/national guard. (Other nations might also have battlemages in their military forces, but we haven't seen any yet.) The only one identified by name so far is Meji's friend Bani, but there are others, as shown here, here and here. Distinguishable from an anonymous guard by wearing robes rather than armor, the better to cast spells, presumably.
We don't know for sure what spells and weapons a battlemage can use. Bani was first seen wielding a Yuuki staff, to Meji's envy. The other members of the Tsuirakuan military/guard (presumed to be battlemages) also carry staff-like weapons, but we don't know whether they're all Yuuki staves.