Dear Miss Bunt.
I had assumed that the Professorship kept you informed about the specifics and motivations of our current arrangement. But perhaps maintaining a clandestine chain of information is important, though I would personally not hide anything from the people under my command. It's a slippery slope to mistrust.
Coming to your question, we are both motivated by what is clearly a mutual threat to our agendas. My stance has always been clear, I am pro-blackmarket. The Legion intends to either destroy it or regulate it rather than leave it the way it is, and even if not immediately this will eventually bring us into conflict with one another.
So if you're going to ask me what I hope to achieve from this partnership, if we can call it that. Then my answer would be that I am choosing to kill the Legion rather than see either of us die for their sake. They betrayed you, they will inevitably betray us and I am obligated to protect the Xeno Alliance from suffering the same fate.
It's just logical. My predecessors might have been content with being the Commonwealth's expendable militia. But I have goals that I intend to see achieved, and whittling away under the selfish regime in Vespucci is NOT one of them. Our efficiency together in Colorado is just more proof that we are strong enough to face this threat together.
There's also the fact that you're infinitely more charming and interesting to cooperate with than the wanna-be politicians and brain-washed grunts of the Legion, so it wasn't exactly a hard choice to make when my options were mutual benefit or exploitation.