Mike and Nancy's little sister? Only thing I can recall with her is seeing the monster in the wall in wills room and I am pretty sure anybody who had been in the room would have seen that.
While there may be a reason that will was targeted, or barb, or Holly's presence drew it's attention I am really really confident El is the only one with a true "connection" to the UD since not only did she open the gate but after a couple rewatches I am almost certain it was more of a blank canvas until she punched the hole which is why it looks like a exact mirror of the real world.
I think El made the first connection with UD, and, as you said, kind of influenced it's looks by punching a hole into it, allow both sides to "seep into" the other.
As far as the monster goes, I feel like it was looking for El after making first contact. I mentioned it before, but it always targeted kids. Sure, it killed some adults, but those feel like examples of "you are in my way". Think back to the final episodes. It killed like what 10+ Hawkins grunts to get to the kids. It didn't take any of the Hawkins guys back to it's world to feed on them. It could've, but it didn't. It was looking for something. It think it was looking for El. It can't distinguish between children, so it just knows it's looking for a small human. Why? Who knows, but I think her being the first one to make contact with it on the other side is definitely part of it.
Yeah, Mike and Nancy's sister. She spends the entire show barely saying a word and is seemingly very easily frightened (based on the way her mother treats her and her reaction to Dustin slamming the table anyway), yet she was perfectly okay with something coming out of the wall to get her. That was suspicious as fuck.
I don;t believe El affected the UD at all. I think it's always been connected to ours. When El made contact I believe she just gave the creature the ability to see/sense things beyond the veil that keeps the two separate.
The creature also took the two guys on a hunting trip so I don't think it's a case of mistaken identity. I actually think there might have been a link between El and the creature that let it sense her. I think it's a bit too much of a coincidence if it just happened to find Will shortly after El left him.