I'm currently in a creative writing exercise of just pumping out as many unique semblance as I can in my Google docs so my current process is taking a semblance and figuring out how it would fit a character, what it says about their personality, etc. and then from that, figure out combat behavior, weapons, maybe history. For a recent example...
I've got a semblance that essentially boils down to them being able to create aura shades of people who pass away in their presence, basically pseudo-necromancy. Now, the obvious connection is necromancy=bad guy, but what does this say about his personality? How did this semblance get awakened? Well, maybe his ability to temporarily "revive" the dead is born from him not being able to let go of loss or guilt, he has an extremely hard time letting go, he'd do acything to see someone he loved one more time, maybe. Okay, if that's what he's like, how might this have awakened? A pretty obvious and not bad idea is that it was awakened when he watched a loved one die, but who? Parents or childhood friends are good but a tad cliché, aren't they? Why not take it to a smaller, more realistic scope, maybe he was a student at Beacon, maybe his semblance has been unlocked for a while but there's never been a chance for it to actually show itself. Maybe he was on his first team mission, or even one further down the road when they don't have a teacher supervising, where one of his teammates who he looked up to, respected, maybe even loved but never knew about his feelings gets fatally wounded and passes away and, in a moment of panic, his semblance activates.
Now skip ahead, how does this paint his world view, not only watching someone he loved die but then seeing them come back and then disappear again since his semblance isn't permenant? Would he feel a sense of closure, would he feel the world is cruel for forcing him to watched those around him die only to see them again and watch them leave a second time, or would he see this as a power that he can use to amass power and become strong? Or, maybe, all three. Of course he feels that closure, seeing the person he loved once more and being able to confess everything even if they can't *really* hear him or respond because it's not really them. He feels the world is cruel despite this because if anyone dies around him, even if he doesn't activate his semblance, he can still feel the aura inside of him, alien and lost and, depending on his belief, maybe he thinks he's keeping them from getting into heaven or hell. He feels this is a way to amass power, anyone he feels is a "bad guy" who dies around him give him the chance to use their power for good or at least his version of it. Three things that can contradict each other but still exist in the same person because they're only human and even if they try to be one thing, the other thoughts will still always be in the back of his head.
So what would his weapon be like? If he turns into someone that kills those around him to gain their power it would be something quick and clean. Easy kills, minimal damage to the body and soul. If he turns into someone who simply takes from those around him, he could focus on anti-grimm weapons and let his shades do his human fighting for him.
Once you figure out one thing you can usually take some part of that thing and extrapolate it to figure out the rest of the character naturally.