@Bleak - Super Toughness works. You got yourself a real scary looking tank there. Your other Stunts can be difficult, so no worries. Since they aren't powers, they need to be just extra little things that represent how skilled your character is at certain things. Each skill has three example stunts on the SRD page, which I'm sure you noticed.
There are a lot of things you could do. Sometimes people focus on their best skills for their stunts making them really good at doing things with those skills. Other times people use their stunts to bolster their lower skills to give them advantages in very specific circumstances. An example I saw once was a character with a low Shoot score in a Weird West setting. He had a stunt that let him use Notice (his best skill) for shooting on the first turn of combat. He justified this as saying his instinct and reflexes took over in those heated moments, but once the gun was in his hand and he had to pick his targets, he was not that great of a shot.
Your highest skill is Rapport, so we will start there. Best Foot Forward is one of the sample stunts. It says: "Twice per session, you may upgrade a boost you receive with Rapport into a full situation aspect with a free invocation." First, we should describe what a session is for us. Normally, it means every time you sit down at your table to start a game, but since we are online, it will be closer to every three or four long scenes. Basically, I will declare when a new scene starts a new session. You can probably count on a new session starting anytime a new day starts, as long as something actually happened the previous day and your character didn't just veg on the couch.
Next we will look at terms. In Fate, you can have four possible results from a roll...Failure, Tie, Success, and Success with Style. Failure in this game does not always mean you fail, sometimes you get what you want but at a price. Rapport will usually be used to Overcome or Create An Advantage. On a Tie to Create An Advantage, you usually only get a Boost, which is a short, one time use +2 bonus to your next action (assuming that action is in line with the first...you can't expect to cuss somebody out on one turn and start a make-out session on the next...most of the time...I'm not kink shaming anyone.) If you Succeed on a Create an Advantage action, you instead create a Situational Aspect. Aspects last longer, usually until the end of a scene unless someone makes it go away with another action. Something could be On Fire as an Aspect, but when someone uses their action to dump a bucket of water on the fire, that Aspect goes away as the fire goes out. When an Aspect is created, the creator, or someone they choose, can typically gain a free invoke (a +2 bonus) to some future roll.
Best Foot Forward allows you to take a tie, the small Boost, and turn it from a temporary advantage into a full on Aspect with a free invoke that lingers for the rest of the scene. As a fence of stolen goods, I could see a scene where a dangerous goon brings you some jewels from a smash and grab. They want lots of money, but you feel it isn't worth the lower profit margin. So you try to Create an Advantage and convince him you are giving him "The Best Prices in Town" given how much heat he's brought down on himself with cops. You roll, you tie. Normally, the goon would be suspicious and need further convincing. You would get a small boost to your next attempt at convincing him, but with this Stunt, you can turn it into a successful Aspect and on your next turn, convince him he should accept even less money so with the promise that your door is always open to him anytime he has merchandise to move.
Perhaps you decide you want to hire him for a specific job later in the scene, you would still have that Aspect to call upon to convince him. But if he leaves and the scene ends, the Aspect goes away, he might learn that Vinny da'Screw over on Baker St. would have paid what he wanted, the goon isn't mad exactly because he got some cash, but he knows you are no longer the "Best Prices in Town (You can spend a Fate Point to turn Aspects into Story Details if you want...making them essentially permanent, but that is advanced stuff)
Other times stunts are much simpler, Demagogue just gives you a +2 when you give a stirring speech to a crowd. No special rules...a simple +2 to the result. Popular is probably not a great one to start with, because you are in a brand new world and don't actually know anyone. You could choose to leave the stunt open, then create it later with a Fate Point. You might take Black Market Contacts that let you know someone in the area once you've explored some.
I hope this helps. As you think more about who Lotte is/was more ideas may come to you. And I'll just say again, you don't HAVE to come up with them right now, you are allowed to create them on the fly if you want. Maybe something will come to you in an ealry scene and you'll think "I wish I had this trick to pull." and you can just declare that it is a trick you know.