Actually, I'd have Nerissa join, and to fill the place as Louis' enemy if that's alright with you- and I'll assume it is by the way you offered up the position so enthusiastically.
Perhaps it could be a case of Louis being in the wrong place at the wrong time; Nerissa has suffered from unbearable headaches ever since her near-death in the Black Lake, and has quite a deal of trouble relating to others. Perhaps she could be set off by a combination of something Louis does, and her pain, and ultimately end up breaking down and attacking him. Otherwise, Louis could have done something in the past that had greatly upset her. For example; I wrote once of a student who had destroyed the plants within Hogwarts’ greenhouse in order to anger Nerissa, who she had then shot one of the unforgivable curses at due to her rage. This position could be filled by you, and in turn, I would be filling the position as Louis’ enemy and would join you in a duel- or "battle" as you called it.
I’m up for other ideas as well, though I have provided you with most of mine.