The primary goal of Keto diet or low carb diet is carb restriction. That means you should limit the foods that contains high amount of carbohydrates like soft drinks, candies, grains like wheat, barley, oats, etc. You need to restrict your carbs intake up to 30g – 40g per day to reach ketosis (a natural way to burn stored fat rather than carbs)
Although fruits are often considered healthy, but they are actually very high in carbs and sugar, unlike non-starchy vegetables. They contain natural sugars like glucose and fructose which will spike up your insulin levels constantly and kick you out of ketosis. So for e.g., if you are eating a large apple (about 25g of carbs) or a medium sized banana (24g of carbs), it will exceed your daily carb limit on keto diet which will destroy your weight loss efforts. Therefore, when it comes to keto diets, most fruits should be avoided.
However, certain berries are an exception that can be enjoyed in small amounts. The best choices are blackberries, raspberries, and strawberries, which provide 5-6 grams of carb per 100 grams (3½ ounces). You have to be very selective when planning to have them on a low carb or keto diets so as to meet your daily macros.
Choose the fruits with the least amount of carbs. You can add fruits like strawberries, avocados, berries to your keto shakes in a small quantity just for a taste while on a keto diet. It is best to stick to those fruits that have a minimal impact on insulin as well as those with very few net carbs.