Beanpole

I just read John Christopher's Tripods series! (Well, for the second time, really-- the first time was in 2012.) They're really good-- especially the first two.

I really like the nerdy French inventor character. His actual name is Jean-Paul Deliet, though the English protagonist, Will, calls him Beanpole at first because he can't speak French too well, and the nickname sticks. They live in a post-apocalyptic neo-medieval culture after an alien invasion, and at one point they do down into an underground passage in one of the "great-cities of the ancients" (aka the Paris Metro) where
they discover a box of iron eggs which mysteriously explode when you pull the rings. Yup, it's a grenade!

So here's Beanpole, with a grenade aka "iron egg." I would have had such a crush on him if I'd read these books when I was younger.