How good is Durin actually for Klee

I know that Durin is very good, but I've been wondering just how good he is for Klee. Where is he on the scale from "notable buff" to "game changing?" I feel unable to find a quantitative description that puts Durin in perspective to other players and to Klee's base damage.

For further details, I've got Klee C5 with Lost Prayers. I've been playing since 1.1 and accumulated a lot of characters. I was originally planning to save for Xilonen and Neuvilette (currently C1) constellations. Would you say that Durin is worth it? Furthermore, I've seen his C2. It seems so strong that maybe the best course of action for full DPS could be to go for that. I currently have enough saved for a realistic chance for C2 and even the weapon.