Before I begin this, I should probably explain who and what River Tam is to Simon. Because while more people have watched Firefly (:DDDD), not everyone has, and some of these people play characters who Simon interacts with extensively. And so it always helps to have a common frame of reference!
River is Simon's younger sister, a damaged psychic who is deceptively fragile in that she can fuck up your shit physically but cries at vomiting on her brother's bed. Simon loves her unconditionally, and gave up his life, profession and liberty to rescue her. Now, this is where things become complicated. In order to continue functioning as River's caretaker, Simon convinced himself that he didn't need anything else as long as he had River. This, of course, was proven wrong by the television series, but Simon reverted back to this mode of thinking after Kaylee left in December. Yes, it hurt that Kaylee left, but at long as River was around, Simon could continue to function. Then River left. Simon is not an optimist. Simon is a realist/pessimist. Before yesterday, Simon believed wholeheartedly that the reason why Death had taken River home was because River was actually dead. This is because the last time River messed with the barrier, she did die, and Simon is quite aware of this.
Now at this point I had three possibilities with Simon. These are:
1. He could continually try to think of ways to escape so he could get back to River.
2. He could realise that this was futile and sink into depression.
3. He could find an alternate way to be sure that River was alive.
Now, while the first would work in the short term, Simon already did this in canon and as we see in the first episode, he is clinical and distant with everyone. This does not bode well for long term roleplaying possibilities. The second would kill off all roleplaying prospects and have the additional problem of not being funny. Because, let's face it. Clinical depression isn't funny, and I'd be doing a disservice to anyone who has known someone with clinical depression or someone who's lived it themselves in playing it in a crack game. Other alternatives included "drop Simon" and "pretend nothing's wrong" but those were lame.
There were really only four people that Simon knows could tell him with any degree of surety whether a girl who was born five hundred years from now and in a different part of the galaxy was still alive: Death, Delirium, Francescu Gargamel and Ichihara Yuuko. Death and Delirium told Simon that River was all right and that he would see her "soon", however Simon's pretty aware that the Endless live a long time and how he defines "soon" isn't how they define it. He was pretty concerned that "soon" meant "she's dead and you'll see her there". Francescu Gargamel told Simon he didn't know. And so that left Yuuko and her shop o' wishes.
To cut a long thread short, Simon received a firefly mark on his arm that will disappear if she dies. How this works is that it sort of creates a link between River and Simon; a link which Simon's lifeforce, energy, whatever, serves as the power source - this was the price that Simon paid for the wish. There is, of course, a problem with this. River is five hundred years into the future and on the other side of the galaxy. One of the conditions of the wish is that the further away Simon is from River, the greater the cost would be to keep the link open. Right now, that connection is extremely draining. Simon knew this, understood it and accepted it willingly. Why, you ask? Simon'll make any sacrifice for River, regardless of the cost to himself.
So what this means is that basically Simon has this bloody long spiritual tie between him and River that drains him constantly. Most people without extrasensory senses won't notice a thing. Simon's good at hiding stuff, most people at camp only know him as "Doctor Tam" and considering that the majority of his time is spent treating people in pain, they're not in a condition to notice! Those who know him fairly well or are particularly observant will notice that he tires easily, he sleeps like the dead, eats more than he used to - but all of these could be attributed to depression too!
However, if your character is spiritually talented, they might be able to sense that there's something really funky going on. For example! The shinigami/quincies may be able to sense that there's something a little off with Simon's "ribbon" thing, but unless they both know Simon and would actually care, then it wouldn't ping them. It's not a Darkness/demon/angel thing so I'm not sure who would sense it. It's mainly a magical/energy flow thing. So if you think your character can sense it, let me know!