So Condemned, while not really that obscure, is pretty old and this guy is only a secondary character. So if you're wondering who he is, what he does, if your character can put him in a box or a soulstone then READ FURTHER.
The Demon is the physical personification of Ethan Thomas' (the main character's) inner demons. Alcohol, acceptance, really poor taste in shirts, whatever.
As a demon he'd be subject to characters that can bind demons or detect them, though he wouldn't be damaged by holy water or anything like that. He's not that kind of demon. He's also technically an induced hallucination so if a character had some sort of mind-control powers he'd probably be diminished somehow or flicker in and out of being. I say technically a hallucination because he is able to kill people and crash a plane and interact with the environment, so he has to be there in some sort of physical sense.
Personality wise he's actually one of the more helpful people in the game. For being a hallucination caused by the Oro intending to kill Ethan, he sure spends about 90% of his time looking out for him and even serves as the in-game tutorial. So he's a pretty helpful guy. In a sarcastic, caustic sort of way.
He can't be killed, but he can be damaged and weakened to the point that he'll phase out of existence and wouldn't be able to materialize again until he built his strength back up.
If you don't mind spoilers and want some videos you can see him:
Berate Ethan for not doing the thing he should have done!
Kill a guy and crash a plane!
Encourage on the job drinking!
Be the only guy who knows what's going on in the whole game!
Powers & Abilities
Summon Mannequins: He can summon immovable unbreakable mannequins which look like this and function like this. In other words they won't move if you're looking at them. And are creepy as fuck.
Endless Alcohol: You heard right. Never ending alcohol he can make out of nothing. Drink up!
Endless Supply of Baseball Bats: Don't ask me why but he can create Baseball bats. Also if he throws them they'll disappear after they hit the ground.
Phasing: He can phase in and out of existence which makes it look like he's teleporting, though that's not really what he's doing. He doesn't have to, but if he wants he can coat everything in ooze when he phases. It doesn't do anything, it's just gross.
TV's and radios: He can posses radios and TV's and use them as 2-way communicators. If you break the TV when he's in it it will actually injure him somewhat.
Telekinesis: He can throw objects with his mind. Nothing much bigger than a desk, though he can rip doors off the hinges.
Sonic Attack: As a duplicate of Ethan he has the same sonic voice ability. Of course he'd have to take off the mask to do it so this is more just speculation.
He doesn't eat or drink, or breathe, he doesn't have a circulatory system so he can't be drugged. (Though he has a central nervous system, if he was injected directly into the spine it would work, but probably have a lot of unintended consequences.)
Limitations
If injured he'll bleed a black goopy kind of tar that looks like this. It's not damaging to other people, but it is pretty gross looking. He doesn't have a pulse and he doesn't breathe so he can't drown or be smothered. If injured enough you'd be able to see that his torso is hollow, he doesn't have any internal organs.
Because he was created by the Oro, he's filled with lots of metal stuff. Take a look at this guy:

Similar to that, but not as fucked up in the face. So because he's filled with all this unusual hollow metal he's incredibly susceptible to sound. He could be completely incapacitated by the right sonic frequency, probably at some high level that wouldn't be audible to humans. He'd also not appreciate being electrocuted. But he's not magnetic.
Mask:
Since the Alcohol Demon was created by the Oro, his mask is ornately carved with Oro symbols in addition to being painted to give him a constant smug smirk.
Ain't he cute?
In the game he never takes the mask off it is actually his face. It won't come off if someone tugs on it or tries to cut it off because that sucker is welded right onto his skull.
Of course this is RP so the possibility exists that there could be an event where he's turned human or the mask comes off for whatever reason. Therefore he has to look like something under there, so I'm assuming that he looks similar to one of the Oro from the game since this would be a good reminder to Ethan of what could happen if he doesn't defeat them. Basically having him look like Ethan under there is sort of confusing in an RP setting. So meet Inferi.
Inferi is the first person you encounter in the game. It's not obvious until you play further or replay that Inferi is actually one of the Oro (note all the fun metal in his mouth?). The reason I'm using him is that when he walks past Ethan in the bar Ethan's drink turns into tar briefly, exactly the same way that it happens later in the game when the Alcohol Demon sits next to him when he's drinking. Probably a coincidence, but good enough for me! Additionally you can see in that pic of the Demon that scar down his throat? Ethan doesn't have that, but Inferi does (I just can't find a good high-res shot of it.) And thus, headcanon was born.
The Demon is the physical personification of Ethan Thomas' (the main character's) inner demons. Alcohol, acceptance, really poor taste in shirts, whatever.
As a demon he'd be subject to characters that can bind demons or detect them, though he wouldn't be damaged by holy water or anything like that. He's not that kind of demon. He's also technically an induced hallucination so if a character had some sort of mind-control powers he'd probably be diminished somehow or flicker in and out of being. I say technically a hallucination because he is able to kill people and crash a plane and interact with the environment, so he has to be there in some sort of physical sense.
Personality wise he's actually one of the more helpful people in the game. For being a hallucination caused by the Oro intending to kill Ethan, he sure spends about 90% of his time looking out for him and even serves as the in-game tutorial. So he's a pretty helpful guy. In a sarcastic, caustic sort of way.
He can't be killed, but he can be damaged and weakened to the point that he'll phase out of existence and wouldn't be able to materialize again until he built his strength back up.
If you don't mind spoilers and want some videos you can see him:
Berate Ethan for not doing the thing he should have done!
Kill a guy and crash a plane!
Encourage on the job drinking!
Be the only guy who knows what's going on in the whole game!
Powers & Abilities
Summon Mannequins: He can summon immovable unbreakable mannequins which look like this and function like this. In other words they won't move if you're looking at them. And are creepy as fuck.
Endless Alcohol: You heard right. Never ending alcohol he can make out of nothing. Drink up!
Endless Supply of Baseball Bats: Don't ask me why but he can create Baseball bats. Also if he throws them they'll disappear after they hit the ground.
Phasing: He can phase in and out of existence which makes it look like he's teleporting, though that's not really what he's doing. He doesn't have to, but if he wants he can coat everything in ooze when he phases. It doesn't do anything, it's just gross.
TV's and radios: He can posses radios and TV's and use them as 2-way communicators. If you break the TV when he's in it it will actually injure him somewhat.
Telekinesis: He can throw objects with his mind. Nothing much bigger than a desk, though he can rip doors off the hinges.
Sonic Attack: As a duplicate of Ethan he has the same sonic voice ability. Of course he'd have to take off the mask to do it so this is more just speculation.
He doesn't eat or drink, or breathe, he doesn't have a circulatory system so he can't be drugged. (Though he has a central nervous system, if he was injected directly into the spine it would work, but probably have a lot of unintended consequences.)
Limitations
If injured he'll bleed a black goopy kind of tar that looks like this. It's not damaging to other people, but it is pretty gross looking. He doesn't have a pulse and he doesn't breathe so he can't drown or be smothered. If injured enough you'd be able to see that his torso is hollow, he doesn't have any internal organs.
Because he was created by the Oro, he's filled with lots of metal stuff. Take a look at this guy:

Similar to that, but not as fucked up in the face. So because he's filled with all this unusual hollow metal he's incredibly susceptible to sound. He could be completely incapacitated by the right sonic frequency, probably at some high level that wouldn't be audible to humans. He'd also not appreciate being electrocuted. But he's not magnetic.
Mask:
Since the Alcohol Demon was created by the Oro, his mask is ornately carved with Oro symbols in addition to being painted to give him a constant smug smirk.
Ain't he cute?

In the game he never takes the mask off it is actually his face. It won't come off if someone tugs on it or tries to cut it off because that sucker is welded right onto his skull.
Of course this is RP so the possibility exists that there could be an event where he's turned human or the mask comes off for whatever reason. Therefore he has to look like something under there, so I'm assuming that he looks similar to one of the Oro from the game since this would be a good reminder to Ethan of what could happen if he doesn't defeat them. Basically having him look like Ethan under there is sort of confusing in an RP setting. So meet Inferi.

Inferi is the first person you encounter in the game. It's not obvious until you play further or replay that Inferi is actually one of the Oro (note all the fun metal in his mouth?). The reason I'm using him is that when he walks past Ethan in the bar Ethan's drink turns into tar briefly, exactly the same way that it happens later in the game when the Alcohol Demon sits next to him when he's drinking. Probably a coincidence, but good enough for me! Additionally you can see in that pic of the Demon that scar down his throat? Ethan doesn't have that, but Inferi does (I just can't find a good high-res shot of it.) And thus, headcanon was born.