I don't usually play badasses, so don't take my suggestions too seriously as I haven't put them to practice much (Catillah can be a bit of a badass at times, but not on most days), but to add to what others have said: emote.
It has been said you have to say little, hear more. Problem with that in channels is that people will easily forget you are there, and consider you afk. So they won't perceive you are listening and not saying, they won't take you into account. So you can emote, proving you are there and listening and, yet, not saying anything.
Emote goes, indeed, along the lines that don't include "how strong your muscles are, which could break a man's neck with no problem" as someone said. Badass, to an extent, is a deal about what he could do, not what they do: they thin, veiled theat. A badass doesn't have to be big or strong, take Habbibal Lecter in Silence of the Lambs, for example, he can certainly be threatening, and yet will always be polite and isn't that big.
It's something you can't actually pin in what he says, and how he moves. You can feel the tension, even if he's just smiling at you from the other side of a table. This is harder to do only in text, of course, but a good choice of emotes can help. Each character has different ways to emote, that portray different aspects of him (for example, old Seph smiled often, Catillah usually grins instead, and that came out without me thinking on it, just happened because they're different), and so choosing tough/dangerous gestures and emotes can do a lot to convey that threatening feeling. Plus a good portrait, like Korsavius'.
If you have friends in that know what you want to RP and can help (hopefully, also, friends with a certain recognition among the community you want to join), let them tell bits of your story. In a way, it's the same that happened to Istvaan, though in a easier way and (unfortunately) also a less powerful one. But there can't be many Istvaans in the world.
And, game wise, you have to be willing to step up. When problems come, issues arise, etc. you have to be willing to take the responsibility on your shoulder and live up to the test. Doesn't matter if you succeed (which would be better) or not, but you have to do it. You may RP a reluctant badass that does it even if he doesn't want to (like the Bruce Willis kind) or one that jumps right into it without hesitation (more like a Schwartzenegger kind), but in any case he has to jump into it. And, hopefully, be in the center of it, one way or another. That's a story that will be told, afterall, and will build up to your reputation and position even when you're not interacting or even in the scene (which joins to what Desideya just said).