ok so how about, his arms and legs CAN extend, like patlabor (one of my fav's). Would kinda make sense, when the main char wants to mount him he shrinks down and then when shes on he extends up. hmm?
I dont like the legs extended. If she wants to climb up, the summon could simple lay out its arm as a ramp. And even though I voted skinny arms I think you should look over the elbow construction since to me it looks like it could break easily. Would perhaps be more charming with the classic cylinderjoints, or perhaps…