I'm gonna start formatting these better to make them easier to read.

Thief: Someone who steals their aspect from others to use themselves. They can do this consciously of their own free will, but it also seems like it comes instinctively for some thieves. The fact they can steal their aspect is significant to them in some way. (Edited from the last time I talked about a thief, I realized I had gotten something wrong).

Doom: Fate (specifically an unfortunate fate), futility, sacrifice, and death. Time has an association with death, but doom directly deals with it. It heavily involves ghosts and the afterlife. This aspect seems to be connected with rules and restrictions. It's usually tied to the specific event(s) in their sburb session that will doom everyone.

Okay! So, let's talk about how sometimes people are inherently screwed over by their classpects before we get into this.

This was elaborated on heavily with Dirk in the comic, he destroys emotions and relationships around him, and his own soul is splintered. While he could hypothetically find a way to hone his powers towards people who deserve it, like when he was attacking Aranea in game over, it's still in his nature for him to destroy those things in general and very easy for him to hurt those around him.

Another example (and perhaps more fitting for this analysis) of a character with an inherently unfortunate classpect would be Mituna, who's also a doom player. He inherited doom, sacrificing his cognitive function, to protect his team.

I also think that some classpects are just not meant to ever mix. That doesn't mean they won't, though. So, let's get into it.

A Thief of Doom would be the most self destructive person in the world.

Something that might make stealing their aspect significant to them in some way is that they were saved from something and they wish that they weren't. Maybe they were an addict and had their vice taken from them, or maybe they were going to sacrifice themselves for something but someone else did it first. Or perhaps they thought they had some awful fate laid out for them that was shattered by the world ending as they entered the game.

If they work alongside their teammates, they'd be self sacrificial to a fault, to the point it's pretty likely they'd only really fulfill their classpect once by sacrificing themselves to save someone from death. However, sacrificing themselves wouldn't mean they have to die, they just very well could. They could also sacrifice something else like Mituna did, such as an arm, a leg, or a sense.

I think the most powerful things a Thief of Doom could do is bring someone back to life at the expense of their own. Thieves don't just steal their aspect, it's always something they use for themselves. They might also save their session from a cataclysmic event that would doom them all by finding a way to save everyone at the expense of their own life.

So, they could possibly end up doing the same thing Mituna did, but sometimes the things that separate classes of the same aspect isn't their powers, it's their characterization.