You know I really never got the hang of "invading" and such in this game.
Today I had 3 very odd things happen.
1. Somehow I turned human, don't know how, but I did. My SL15 character in undead got ganked by a guy with an extremely higher level.(though any level at that point is pretty high by comparison.).
2. Then someone joined and me and him talked and he gave me a couple of items(though half of them fell out of use within 30 and me upgrading my weapon). Honestly my first invader who was actually cool!
3. Finally, on my (slightly) higher character I was going to kill the gaping dragon. Summoned a few buddies there to help me. a) Someone invaded only to be ganked by 3 people at once. It was pretty funny. b) The highest guy(must've been pretty high) was doing some nice dmg vs the dragon, but he then proceeded to get grabbed/killed. and we were left there. c) The other guy was so low leveled I was doing most of the dmg by the end and it was just silly how much hp that thing had. But its a pretty easy boss comparitavely, so it went fine in the end.
Anyway COOL STORY BRO, but that was my day in Dark SOuls.
You can't be invaded by Darkwraiths that have a higher SL than your own. It's not hard to get good equipment at a low SL though. My SL 12 character that I play co-op with is in NG+ and have basically every good weapon you can imagine.
Yea, like has been said, you can only be invaded by and summon people who are within 10% of your level range. There's lots of ways to make an overpowered character without being very high level as long as you have the knowledge and/or skill.
In Undead Burg, sometimes there are the kind invaders who only invade just to help out the newbies.
In the Depths, the guy who invaded you was probably just an NPC invader; he will always invade in there and is extremely easy to kill.
About how you magically turned human... doubt that's possible, you must have reversed hollowing at a bonfire without realizing it.