[He may have gotten involved by sheer coincidence, but the phrase "you won't be able to turn back" makes up his mind for him - there's no way he's staying behind.
That said, he's far from comfortable with this. Not knowing where they're going or what lies ahead, what this is all about, it scares him like it's always scared him. Still, he doesn't have a choice. He needs to do this, needs to know because if he can just find out everything there is to know about Marina he can work this out. That's the way it works.
Familiar faces are still welcome, seniority even more so and without a Hotch to fall back on he goes for the next best thing. Voice low and cautious he makes his way over to her.]