Being ISFJ, shouldn't you have that natural tendency to appear strong and confident? My ISFJ sister is pretty scary and dominant. She's adjustable too though so it makes her a bit paradoxical.
No, that's ESFJ, most likely.
I think it's an ISFJ trait to be considered as "mostly harmless".
I think the main source for that trait is ISFJ's tendencies to uphold the harmony at the expense of oneself.
I know few ISFJs very closely, and I've noticed that each one of them is completely and utterly incapable of saying: "No."
They are more inclaned to try to redirect the conversation in such a way that their opinions won't be decisive or against the majority. They especially won't go against people close to them.
Try cooking an ISFJ a dinner, and ask his or her opinion of it. No matter how terrible a cook you are, they are never going to say "That was an awful dinner. You shouldn't be cooking". Instead, they'll appreciate the effort and thought behind the deed.
So whatever you do to them, they'll just smile and say "Thank you". Well, not always, but you get the point.
So if you want to get rid of your "mostly harmless"-tag, this is what I think you should be doing:
-Become more objective, rather than subjective. Giving negative feedback is actually more useful than only giving positive feedback. Giving only positive feedback rarely leads to concrete improvements. It sure gives a morale boost, but it can also contribute to delusions of grandeur. Hence one should consider using a "hamburger"-model. That is... first give positive feedback, then negative feedback, and then again more positive feedback. This way you can actually deliver critique without seeming like an a-hole.
-Know your rights. It's not selfish to stand up for yourself. For example, should someone jump in line just in front of you, don't think "We'll, they sure are some rude people... they'll get what's coming for them sooner or later". Well, the truth is... they'll get that, and much more. Laws of karma is not really a real thing, so don't count on it. Make sure you get what's yours, and that no one else is taking it. So... the next time someone tries to jump in line, raise a hell and make it damn clear that you are not tolerating this at all. Don't be afraid of making a scene, when you should be making a scene.
-Voice your opinions, and expect others to do so as well! There's no such thing as "normal". Don't be so worried about that people will find out that you are not normal, because no one is normal! We are not the same, and each one of us has their own way of living. Don't expect people to know what should be, because their view of "what should be" is most likely very different from your view of "what should be". I've come across quite a many times when ISFJs are frustrated by other people who do not know how to behave themselves. However, almost never these ISFJs choose to act because they think that everyone knows how to behave, and that people behaving badly are just evil from inside. In reality, those people just are not aware that they are insulting you with their behavior. So... make sure you raise your opinions, and make sure you also take into account other people's differing opinions. They are not there out to get you for your "weird" opinions, they just have a different perspective of what's normal to them.
Well... that turned out to be a rant.
Short summary: Acknowledge that you are worth other people's time and attention. Acknowledge also that people don't read your mind and don't always know you are being offended. You have to say that out loud. People are being treated just the way they accept to be treated.