Mistoressu is a term for a fictional character, usually in anime or related media, that someone has sexual, and sometimes romantic, affection for but is not in a close relationship or marriage. Mistoressu has it's origin from the word mistress and sounds in Japanes like Mistoressu.
mai mistoressu is yoko from the anime...
It is a union of two romanian numerals where the first one is "VI" and the other one is "IX". The latinum equivalent is 6 for "VI" and 9 for "IX". It makes a number of 69 and stands for the sex postion 69.
"I want VIX with you."
or
"Let us VIX right now."