When you forge the last sword, Senji Muramasa is very touched for achieving it.
In Kisuke's second ending, he successfully frees Momohime from Jinkuro's control, and acts as her personal servant in memory of her sister Torahime. This contrasts what Kisuke does to Momohime in the latter's second ending - he kills her as he dies.
Ippondatara fights you because he wants to protect his hot spring from humans. When you defeat him and spare his life, he allows you to rest in the spring, and you can see him take a bath in boar form.