I just caught sight of this beauty in the Dictionary of Medieval Names and it turns out this female name was used on someone with a bit of a pedigree. Brunissende de Comborn was the daughter of the Viscount of Comborn in France in medieval times. However, the first ruler, Archambaud, was of Merovingian descent. There are at least two named Brunissende in the family tree that I can see - Brunissende de Limoges, married to Archambaud the Bearded (1147) and from then on called Brunissende de Comborn. Her name was also written Brunissent, Brunicenda, Brunissen and Brunicens, but her birth name may have been Humberga. Delightfully, one of her daughters was named Melisende. Brunissende de Thiern (1235 or 1318) was daughter of Archambaud VII. Her name has also been written in the Latin Brunicendis.
Are those the only namesakes for this rare French name? Nope. Brunissende of Cardona was another, wife of Roger IV of Foix, the son of Roger-Bernard II the Great. Their son named one of his daughters Brunissende as well - she married Elias VII of Perigord. Then there's the daughter of Vicomte Pierre II de Grailly, born 1315, and also Brunissende de Lautrec, from between 1325 and 1379, daughter of Amalric, Vicomte de Lautrec. So, very historical.
But that's not all. Brunissende is also a character in an Arthurian romance written in Occitan called Le Roman de Jaufré. The tale is of a newly knighted squire who goes on a quest, meets a young maiden named Brunissen, and marries her in King Arthur's court. Interestingly, name-cousin Belisent is used as a character name in other Arthurian romances, including Idylls of the King. Their other name-cousin Melisande is a literary appellation as well.
Brunissende can technically mean "brown, tan," the same as Bruna, Brunhilda and Brunelle, in modern Latin and French. However, it also means "shield strength" or "strong protection," from the ancient Germanic brun and the same element used in Millicent (Melisende) and Belisent (Belissendis). It is a rare name even in France, where only three births were recorded in 2013.