What most people see when they look at Klaus is a carefree, hedonistic, deadbeat junkie. In a sense, this is partly true. He's pretty hedonistic, sure, and his lifestyle is heavily implied to have long been involving doing whatever it takes to scrounge up enough cash to chase the next high, never settling down for long and taking advantage of anyone who so much as accidentally offers help. He doesn't drive, doesn't work, doesn't pay if he can help it, and he gets around by mooching off of anyone he can.

To those that are familiar with this kind of life, it's clear that Klaus is definitely not quite as carefree as he seems. The drugs he uses are to numb himself from years of abuse and trauma, not to mention his power to see the dead that he's terrified of. And that's even before what he goes through during the series proper. He already had a form of PTSD, but it worsens and compounds over the course of the season, and he starts to more regularly experience flashbacks and breakdowns. He copes by self-medicating and self-deprecating and just saying whatever he thinks might be the funniest thing in that particular moment. Seriously, though, he does not shut up, even in potentially lethal situations. He annoys the shit out of his own kidnappers.

Of course, the fact that he uses humor to deflect from anyone looking too deeply at him has the awful side effect of... no one looking too deeply at him. No one takes him seriously, even in cases where he's being logical, because no one expects an honest or deep thought from his mouth, and he has no practice in being openly vulnerable. He's a little sensitive about this, despite the fact that it's an effect of his own making; he frequently spirals because he feels like no matter what he does to try to turn his life around or improve himself, no one will care or even believe he's serious about it.

The thing is, he can be surprisingly competent and sharp if the situation calls for it (and he's not high out of his mind at the time). For example, when Five brings Klaus along to interrogate someone, solely using him because he's the one least likely to ask any questions about why he needs to pretend to be Five's dad, Klaus ends up being the one turning the situation into their favor by taking charge and using unconventional methods to pressure the unfortunate target. Five seems to even be impressed for a moment, at least until Klaus opens his mouth again and pure idiocy comes out. And then, of course, he survives the front lines of the Vietnam War.

Klaus is clearly a mess of contradictions in eyeliner. He's selfish, obviously, doing whatever he can to look out for himself, but he's equal parts compassionate. Flashbacks to his childhood make it seem like it's more in his nature to be a source of comfort and a kind person in general until he grew to learn to be selfish out of necessity in order to survive. He's shown to be surprisingly empathetic when it comes to the pain of others, though this doesn't always manifest in the nicest way. Sometimes he just uses it to lash out in an argument. His one fatal flaw is his cowardice, which prevents him from a lot of things in life, missing opportunities and hardly realizing any of his own potential.

But most of the time, when left to his own devices, he's a victim of his own impulses, he's reckless, he doesn't listen to anyone, he's attention-seeking, and he can't seem to make himself care about anything up to and including the literal apocalypse. Basically, he's got a long way to go, despite the fact that he's sort of working on improving himself.

Klaus has the ability to see the dead. Normally, he can only speak to them, and they are completely incorporeal. There are some suggestions that a dead person is more likely to manifest if they have some connection to the area, the other people around him, or to Klaus himself. The most obvious example is Ben, the ghost Klaus sees most often and most easily since they were close in life before Ben's death.

As the series progresses, Klaus finds that his aversion to his own power has avoided its full manifestation. He starts to be able to physically interact with spirits. Eventually, he can even make it so spirits can interact with others. This happens very occasionally and Klaus has very little control over it, and it only seems to happen in tense situations.

One important aspect of this power, however, is that when Klaus self-medicates with drugs and alcohol, it numbs him to the point of the powers not working entirely. Even if he starts to sober up, as long as any intoxication is still in his system, the only ghost he can clearly see is Ben, due to their strong connection.