That would be the mirror modifier that I hate ;D

There is also a symmetry thing in the top right corner by default but that only mirrors transforms, it won't mirror topology modifications like cuts which really annoys me >_<

Clothes are fine, multi-layered clothing can be a pain. How you'll want to animate them depends on your style. If you have a cartoony style it's easier as you can just weight them to the character rig for big movements and use a clothing rig and maybe shape keys for smaller flaps and things.

Because I'm using particle hair and kind of needed the clothes to look as good I ended up using cloth dynamics x_x which I'm still struggling to figure out. I made a super quick video (link goes to my blog not the hive blog) with my really fast tmoving character and if I can get the cloth dynamics to work with her (they almost did XD I didn't want to play around with it too much longer as I still have so much to build) then they will work with anyone XD