>>50421773this took me far too long to make, but here it is.
The basic idea for this gym leader is that she specializes in "shelled" pokemon. Every member of the team has a shell of sorts or is encased in something, and I thought that was a really cool theme. If I must stick to a type, it would be very easy to pivot into a water type gym leader by using things such as clampearl, carracosta, cloyster, omastar, and so on.
She's this experienced, pretty badass diver and explorer. Been around, did everything from rock climbing to deep sea diving and probably still does some of it on the side. Shell pokemon are resilient and that's what she prizes, you gotta be able to take a hit and go with the flow to get out of adventuring alive. The battle would definitely be one of attrition because the whole team is made to take a fucking minute to go down.
The gym puzzle would probably involve going underwater by using Dive, in an underwater ruin-like thing a bit like Undella in BW. Inside there would be a bunch of giant clams that the player could interact with; some would have pokemon inside, some would have gym trainers, and only one would have a "treasure pearl" that the player had to present to go to the next level of the gym. Upon finding 2 pearl, one blue and one red, the player could get to the gym leader herself.
The keystone would be on the back of an old compass the leader always uses to go adventuring.
I know the team is a bit schizophrenic with the pokemon being shuffled around a lot, but there's just so many cool shelled pokemon that it would be a damn shame to not make use of the variety.
