Yep, 100% completion. Well not exactly. I've got every trophy but there are still things to create with alchemy and everytime I play through, I discover some new mission that I didn't get on the last playthrough.
Goh and Kinu only show up on a side mission. You have to deliver something to them from Zaji and that's it. He doesn't show up in the story at all. I guess that makes him your ally since you're working for the Asuka. You unlock their skins though.
There is no mission editor sadly.
Enemies are chumps until you play on Deadly Mode. Then they get super stats and really make you work.
You can unlock different outfits. Both characters have four unlockable outfits. You can't unlock new swords but the weapons change color along with your outfit. For example, Zen's Outfit 3 gives him a black suit, glowing red eyes as well as a red sword. There are also 46 skins to unlock.
Mimes, Ninjas, and Cholesterol: The Silent Killers