Lempi lands on the "vintage trendy" category: it became first popular in the late 1800's/the early 1900's and is trending now a century later.
Sisu, on the other hand, is a modern word name, becoming popular in the 2000's/the 2010's.
Sisu as a word is very significant to Finns, you can read about it here:
https://en.wikipedia.org/wiki/Sisu (Also,
Sisu is my cousin's MN: his full name is
Topias Kaarlo Sisu.)
I like
Virva, especially its connection to the Finnish folklore and will o' the wisps ("virvatuli" in Finnish). It isn't a very common name, so I'm always delighted when I see it used.