Felix hasn't been, in large part because I want to keep the option of playing him from another point on the timeline. This may change once his younger self finishes canon and gets anti-Bound for several years.

Kain has hardly been in, let alone Bound. There's nothing stopping him, but I can't think of a good reason to do so. "Because I can" is a reason, but it currently doesn't seem like a good one.

Fluttershy also hasn't been Bound, and again, there's no real reason to do so. Even if I had a reason, I probably wouldn't go through with it while episodes are airing, unless there's a significant string of episodes without her or my canonmate(s?) and I make arrangements for millitiming.