My win/lose stats are always reset when I look at it. Does this reset monthly like the high scores? Also, how does fighting event battles (ie Kiyo) affect my stats. Obviously, with Kiyo, I will inevitably lose. Will this negatively impact my stats?

It was reset in May when the cap on stats was raised. The last time before that was years ago.

Nothing will negatively impact your stats, however your losses against the boss will show on your record. Be proud of them, they show how hard you tried!

thanks. I don't battle very often. Actually only when there's an event do I battle.