Talking of titles, the Princes of Monaco are commonly referred to, and addressed as, "His Serene Highness" or "Her Serene Highness". In French it is "Son Altesse Sérénissime". There is a discrepancy here because "sérénissime" does not mean serene, but "most serene" (a superlative).
The word "serene" comes from Latin "serenus", an adjective meaning "cheerful, quiet, relaxed".
Serenus is also used as a first name, and in Ancient Rome it was used as a male third name, as in "Joe Bloggs the Serene", a fairly laid back character ......
There must, of course, be a female variant, and so we have "Serena".
The superlative of Serena is Serenissima, and so we come to Venice, that Most Serene of Republics - La Serenissima !
Finally, an 18th century German writer invented the character of Serenissimus, a rather silly princeling of a tiny principality!
I rest my case....