I have recently decided to start reading up on the X-Men comics, and have noticed a character known as Doop in several issues. Is he an alien ally, an X-Man, or what? A couple of the pictures I have seen have shown an X symbol on his body, but I'm not quite sure if this ties him to membership with the X-Men, or not. Is he strong? Weak? Somewhere in-between. Any help would be useful.
Doop is basically a running joke character; in-universe, he's incredibly strong (he defeated Thor one-on-one) and has a wide range of mental and dimensional powers. He used to be affiliated with X-Statix, but since they were destroyed he's basically unaffiliated. He just knows a lot of the X-Men personally.
Doop was originally part of the rebooted X-Force team, which (unlike the original X-Force) was a spoof of a mutant hero team. They were quickly renamed X-Statix, and were a reality-show style hero team. Doop was the videographer: his job was to follow the team around and record their heroism, which they would use to make the team more money. He was also an unofficial mascot and became close to some of the team members.
Since X-Statix was basically a joke comic, Doop's actual origins were never specified, though he's obviously a take-off of Slimer from the Ghostbusters. He speaks in some unknown language that, somehow, everyone knows, and he has very odd powers. He has access to an alternate dimension which he can send things to at will (at one point he sends all of X-Statix there), and his brain is apparently incredibly powerful. At certain points during the X-Statix run, it seemed like Doop had a secret agenda but they never really went anywhere with it.
Doop eventually "died" with the rest of X-Statix at the end of the series run:
However, as usual he came back to life somehow, and spent a lot of time fighting with the X-Men. Since then, he's made a number of appearances as a "comic relief" style character (often trying to sleep with someone).