Basically the enemy counterpart of Express Cat, Awakened Bahamut or Sanada Yukimura, Brollow flies at extreme speeds, has low health, and deals high damage that can easily break through your defenses in numbers.
Brollow's high knockback count and low health makes groups of them deadly, as a few hits from meatshields like Mohawk Cat and Manic Mohawk Cat can easily knock them back. Take note that they don't have an actual attack cooldown, they only have delayed attacks due to their backswing. Thus, the more they get knocked back, the more damage they cause to your defenses. This makes fast-attacking, low damaging units like Manic Lion Cat much less favorable.
Avoid using the default Cat Cannon against Brollows, as it will cause them to all attack again (think of Awakened Bahamut or Sanada Yukimura being hit after they land a blow). High numbers of hits per second is very bad against them, but powerful single blows from semi-spammable, bulky and heavy-hitting cats like Jamiera Cat, Island Cat and Crazed Whale Cat work well against small flocks of Brollows. However, for large flocks of Brollows, more expensive area attackers will be required, such as Ururun Cat.
Incidentally, D'artanyan's blind spot does not include Brollows, who will rest at the 150 mark instead of 140. This means that, in theory, a D'artanyan can obliterate Brollows, though it is better to have meatshields take hits instead of D'artanyan.
Kotatsu Cat, Kai or Sodom can help a lot as walls to gather many Brollows in the same area to kill them all at once.
Brollow is the second fastest enemy in the game, beaten by Assassin Bear.
Brollow takes inspiration from several different birds:
He is mainly based off of the swallow, a type of bird well known for its black coat and v-shaped tail. Swallows are also a symbol of good luck, fidelity in marriage and fertility in Japanese culture.
He closely resembles a gannet, known to be the "missile bird".
He is additionally based off of the common swift, the world's fastest bird in horizontal flight, which can be clearly seen as Brollow moves forward. It also has a shorter, smaller v-shaped tail, though it is still noticeable. Another similarity between Brollow and the swift: the wings are slightly more forward on the body than they are on a swallow.
Brollow makes a cameo in a spinoff game called "GO! GO! Pogo Cat!", along with Pigeon de Sable and Ecto Doge. Brollows can be seen flying in the background after the player passes the tenth checkpoint.
Brollow and Owlbrow have the same letters in their name but have different amounts of those letters.