Gretchen Wetzel of Criterion Kooikerhondjes wrote a fantastic article that summarizes Kooikerhondje temperament way better than I ever could, and you can read it here!
Kooikerhondjes are spunky and their temperament quirks are very unique. Every individual is unique but generally they are alert, loving, loyal, and biddable, but they are also very sensitive to the world around them and can have a short fuse when interacting with other dogs. When out in public they are notably indifferent to strangers. Even Kooikers who love saying "hello" to strangers will usually only perform a friendly greeting or sniff, and their attention is quickly redirected back to their own person. They bond strongly with their family and, due to their alert nature and history as an all-purpose farmdog, make a functional watchdog for their household.
It is typical for the breed to exhibit some form of sensitivity in temperament. On average they don't like strange dogs coming up to them, and these types of interactions can cause them to snap, snarl, or make growly noises. They don't tolerate weird behavior from other dogs, and they aren't afraid to speak their mind despite their size. They are also not fond of toddlers or small children due to their sensitivity to loud, shrill noises, and unexpected movements. Kooikers should not be marketed as "perfect family pets", but they can definitely live among children as long as their humans are aware of their quirks. These behaviors start to appear in adolescence, and it is extremely important that new Kooiker owners are aware of what to expect and how to properly socialize their puppies throughout the first year and beyond.
Their behavioral quirks come as a reaction to their environment, and should never be unprovoked or out of aggression. An aggressive or overly-shy Kooiker is not typical (but unfortunately not uncommon with bad breeders),
and absolutely no reputable breeder would breed them due to these issues being genetically linked. It is important that any reputable breeder be honest and up-front with puppy owners on what to expect and look out for as their puppy matures in order to set them up for success. Any breeder that sells their puppies as "perfect family pets" or without any mention of the breed's temperament quirks (both good AND bad) does not have the breed's or your family's best interests in mind.
This isn't a breed for everyone, but if they are the right fit their loyalty and love are incomparable. A well-socialized and well-bred kooiker from a reputable breeder makes for a wonderfully loving and active adventure buddy.
If you have a kooiker, please contribute to the behavioral data study by filling out a quick survey HERE!