The first time Will spotted the Man, was shortly after he got back from that place.
He'd been laying in his hospital bed when his mother left to go tell the others he was awake. As Will turned to look as she left, he saw Him standing outside.
The Man was thin, almost skeletal, with pale skin nearly grey in color, and possessed unnatural green eyes. The Man was wearing a dark blue suit, a black tie, and carrying a briefcase. Everything about him screamed "government." Upon realizing He'd been noticed by Will, the Man straightened his tie, turned away from the window, and left.
Later, they discovered that a mysterious "benefactor" had paid everyone's hospital bills, but more mysteriously, Will was back to normal within the week, though it should've taken a few months, at least.
The doctors still couldn't explain it.
When Mike saw the Man, it was shortly after he'd been sent to the Principal for behavioral problems in class. Problems that had arisen more than once.
The last time this happened, the Principal had made it abundantly clear that Mike only had one last chance before more drastic measures would have to be taken. Like suspension.
As Mike moved to enter the office, he could spot the Man conversing with the Principal. No, not conversing, arguing. And it was a pretty heated argument at that.
What they were arguing about, Mike couldn't say. Their voices were muffled by the thick wood and glass of the walls and doors, but Mike was able to tell that his name came up more than a few times. After a solid thirty seconds of eavesdropping, the Man turned around, and looked Mike dead in the eyes.
The Man nodded one last time at the Principal, before exiting the office, and disappearing down the hallway.
Mike later realized that despite his constant problems after that moment, the threat of suspension never came up once. He wondered if the Man had anything to do with it.
When Dustin ran into the Man, he literally ran into the Man.
Troy and James decided they were having a bad day, and decided to take it out on Dustin. They tried to corner him shortly after school let out for the day, and Dustin was forced to break into a desperate sprint to try and get away. When he turned a corner, he collided right into the Man.
Troy and James followed close behind, but stopped when the Man affixed a stern gaze on them. The Man then very calmly told them that their bullying days were over, and that he would be notifying both of their parents, as well as the school's Principal.
Dustin and the others didn't run into Troy and James for almost a year afterwards.
Lucas never really encountered the Man. Spotted Him watching from a distance, sure, but as for actually coming into contact with Him no.
One night, however, while riding his bike home during the storm, Lucas noticed a shadowy figure standing nearby.
Stopping to get a better look, lightning struck merely a second later, about a meter in front of Lucas.
Had he not noticed the Man, Lucas would've rode right into the spot where the lightning hit.
Lucas felt an ominous shiver, but shrugged it off, and continued riding home.
When Eleven encountered the Man, it was shortly before making contact with the Demogorgon.
The Scientists guided her to the Bathtub, and proceeded to hook everything up, while her Papa watched, and the Man stood right next to him.
A few moments later, she was told what she needed to do. The thing she kept seeing, while she was in the Dark Place, she was supposed to talk to it somehow. Learn what it was, why it was there, how it got there... it was a very important task.
But as she was put in the Bathtub, Eleven could see the Man turn to her Papa, and He calmly spoke four words. Four words that would be chilling to her very core every time she heard them.
In His raspy, unnatural voice, the man had said:
"Prepare for unforseen consequences."