According to the Wiktionary, a wet is “(colloquial, derogatory) a weak or sentimental person; a wimp or softie”. TIL.

A free life is, apparently, feeling obligated to go sleep in a tree even though it’s raining and you’ve got a nice comfortable house nearby.

As for the second question, you’ll be better off reading Moomin Begins a New Life (1/62), etc.