Ok... so Boku is Kougai’s beast-servant, not Ririn’s but last chapter Ririn called for Boku and he said it wasn’t her fault but his... narration also mentioned sharing the burden of lies as a fleeting moment of comfort from solitude... so my theory is Sogetsu caught Boku and Ririn being romantically involved and since that’s a HUGE af taboo, Sogetsu was killed off and brutally mutated to mislead investigations.
Also, Rangetsu and Teiyou are so cute