Alright. Last try.
I went over EVERY major NPC and came up with my final possible solutions, each with an explanation. I'm 99% sure one of these is correct.
If the weapon is fleshweaving, it's very hard to resist, he didn't use it for fighting, and Roirr used it too. He also pretends to be friendly.
The weapon could be animals/creatures, which Aria also uses. We first thought he (it?) was just a monster, but turned out more than that.
If the weapons is the box of potions he carried, it's hard to resist, many others used potions, and he's kind of friendly.
The weapon is the wind orb, Drakath also used it, she didn't use it to fight, she fights good herself, and is friendly.
Raven: However, Thursday can resist it, so it's not something that "nobody can resist".
The weapon is lycanthropy. It's hard to resist, she didn't use it to fight, and Chaney also 'has' it.
The weapon is the ultimate orb, which Drakath also used. He might seem friendly when selling doom weapons.
The weapon is science, which Lim also uses. It could also be Warlic's power (which the professor didn't use to fight). Also, at first he wasn't friendly.