Mercury

Mercury used to be a mostly ignored (if terraformed) background world of the Solar System, inhabited mostly by various extremist groups that decided to live there on their own, away from prying eyes of the Solar Commonwealth's general population. It was treated like a stereotypical place inhabited by weirdoes (and remained an object of many jokes).
This changed after the Blackout, as Mercury managed to avoid being decimated by it. As a result, it was almost instantly elevated to the first league of the Solar System powers, something that its planetary government is still struggling with. This has been made even more extreme with the rather complicated issue of its new moon, Persephone, and its alien inhabitants.

Geography

Mercury is a mountain world - massive mountains and equally massive depths are its geography's defining measures. The highest mountains approach 10km height. Between them one can find numerous semi (or not so semi) isolated valleys, some large enough to have their own, small seas, unconnected to the general hydrosphere of the world.
There are no oceans here, only numerous lakes and seas. Plus rivers, but one can expect those to be extremely varied in outflow and greatly dependent on rain cycles.
For now it doesn't seem to be very noticeable, but the hydrosphere shows signs of moving north and south, away from equator. It is believed that within a century or two large swathes of Mercury will change into deserts.
Another major issue to be mentioned are rather extreme weather patterns caused by the air currents altered by the mountainranges and the height of the atmosphere. Expect extreme temperature shifts between day and night and horrible storms, forcing the inhabitants to construct their homes mostly underground.

Ecosystem

Relatively scarce and struggling. Near the poles, liveforms imported from terran tundras thrive relatively well, and the same can be said about the creatures inhabiting local seas. The temperate regions are mostly limited to lichens, weeds, grasses and so on, with forest typically failing to settle in.
There equator region has an ecosystem mostly imported from terran Sahara, which appears to be more or less doing well, at least in some areas. As a whole, this isn't a very nice place to life.

History

Mercury has been terraformed and colonized during the Golden Age of Mankind, soon devolving into the background world of rather low-importance. With its surface divided into numerous 'pockets' by its impressive mountain ranges, Mercury soon become the place inhabited by various groups that seeked to live in isolation from the greater world, yet still managed to get themselves a place on an actual terraformed world.
Its state as the background world meant that it remained relatively untouched by the Transhuman Wars (which decimated the Outer Solar System especially), and the Blackout (which, once again, caused less damage the closer you were to the Sun). This means that while Mercury still has a rather low population as a whole, its infrastructure wasn't decimated and its inhabitants weren't so thoroughly traumatized by losing significant part of their numbers in unknown ways.
Today, Mercury is a bit of a chaotic mess, with its planetary government being initially established as a mediator between numerous self-sustainable and isolationist communities (plus to maintain the solar mirrors that render the planet habitable and police the space of the Mercury Subsystem). Now it somehow become the government of probably the least decimated and ruined economy of the Solar System, that's both mostly self-sustainable in terms of agriculture but also a notable industrial powerhouse.

Mercury
Summary

Distance from Sun: 57 mil. km
Diameter: 4879 km
Solar Constant: 13.196
Mass: 0.055 Earths
Mean Density: 5.427 kg/l
Orbital Period: 88 Earth days
Day Length: 116 Earth days
Rotation Axial Tilt: <10 degrees
Avg. Temperature per Latitude
Poles: -44 C
75 deg: -10
60 deg: 2 C
45 deg: 12 C
30 deg: 20 C
15 deg: 27 C
equator: 33 C

Astrography
Type: Planet [Terraformed]
Location: Mercury Subsystem
Satelites: Persephone
Status: Colonized
Population: ~102 mln.

Governments
Mercurial Confederation