Werther is Professor Hugo Fairweather's assistant, who accompanied the Professor's expedition team to the lost isle and escaped alongside the others. During the expedition he didn't talk a lot, but later after returning to the mainland he turned into a quite talkative JubJub.
Hmm, Kreludite? That's a Sloth toy for you - dangerous but powerful.
Essentially, Sloth is using it to - How shall I say it - MUTATE us.
He had a brief cameo in Chapter Seven of the Return of Dr. Sloth Plot as a scientist, it is possible this means he is an agent of the Resistance.
