I have somehow learned that kostilus means 'perhaps'. Is that just plain wrong or does it mean both things? Like how 'issa' means both "yes" and "he/she/it is"
Well to be precise, "issa" doesn't mean "yes". It only means "he/she/it is". It can mean the affirmative like "yes" but it doesn't mean "yes". "Kessa" means "yes".