This is why campaigns shouldn't be vaulted...newer players miss out on context. Cayde was a D1 NPC who was the Hunters' Vangaurd mentor (Ikora is the mentor for Warlocks, Zavala for Titans). He was usually the one who would avoid debates of what to do in favor of taking action on his own. Bungie turned him into comic relief in D2 until they killed him off during the opening mission of the D2 year 2 expansion, Forsaken. Thankfully, you can at least play most of that mission now (access it through the timeline I think).