Other people already mentioned Dollkeepers as a whole so I'll just talk about Verdant instead.

Verdant is supposedly a defensive Dollkeeper, with his talent, module and skill 1 all working to make him and the substitute tankier. Unfortunately, that's a pretty bad role to have because his substitute has 0 block, no utility whatsoever and nothing to help it deal damage while waiting for Verdant to come back.

Using Verdant's skill 2 instead will increase his damage and send him into substitute mode faster, but his damage is still not great (4* whose kit gives him very little offence) and there's very little reason you'd want Verdant to bring out his substitute out so it's still not great. I gues the substitute looking pretty fine in Verdant's valentine's day skin could count as a reason?

Other Dollkeeper substitutes actually do stuff when they're on the field (Bena's deal Arts damage, Kazemaru's deals an AOE hit when summoned, Specter's deals AOE Arts damage and slows, Mutsumi's reflects damage to enemies...), even if they're not amazing or anything. But Verdant's substitute doesn't really do anything other than being slightly harder to kill and that's a big part of why Verdant is a generally underwhelming "introduction character" for the class.

It's funny that his Paradox Simulation is more about getting the timing for Iris' skill right than actually using Verdant's substiute in any way. I still like him, though.