I see there's space for more than 1 costume slot, how do I get more?

You get one around 17 or 18, you get another one I believe once you join a SG. I believe there are more later on but I'm only 20 and not sure yet.

As you level you unlock more, the first one gets unlocked at level 15. There's also one for your SG

Thanks! ^^

I was just wondering this. :D

First base costume at level one (creation)
Second at 15
Third at 30
Fourth "SG" costume for being a member of a guild/supergroup. It is unlocked when you join, and you keep it even when un-guilded. When you see somone shouting out looking for fillers to help start a SG, helping out for a few minutes will net you a fresh costume slot if you've not done it before! I'm not sure if there is a minimum level to get this one, but I don't think there is.