The Bruh Within fills a crucial ecological niche as gut microbes, helping to break down the meals ingested by their hosts, which are all currently the descendants of the Arctic Legobox.
The Bruh Within can trace their origins to ingested Bruh Cells that possessed a mutation allowing them to survive being digested by legobox fauna, being one of the few spite-immune fauna at the time of their evolution. Life inside a legofauna’s digestive system was actually pretty solid, and with the constant intake of organic matter to subsist on, there's little ineptus to actually leave. As they proliferated in descendant legofauna, genes coding for advantageous traits were spread between populations by way of horizontal gene transfer, diverging further from their ancestry, which resulted in them diversifying just as much as their hosts, if not moreso.
Species of The Bruh Within can vary greatly in terms of overall cell shape and distribution of cilia - which have developed independently across multiple clades of bruh; nevertheless, they are still easily identifiable as bruhs, as all species retain the distinctive bruh “facial” organelles.
Due to the shared immunity to spite among legofauna, they’re fazed little by the occasional surges of alcohol-like waste spite produced by The Bruh Within.