of course kanao saves tanjiro by giving him the serum because of two prevalent themes in kny which are: love and collectivism
many people point out how their dynamic is symbolized by tanjiro who is water and sun healing kanao who is a flower and encouraging her to grow but it also goes the other way. by tanjiro reaching out to kanao and urging her to find her own voice, he starts her on a journey of finding and expressing herself, with everything she goes through in the battle against douma and by the end of the story her will and desire shine through and with that same will she decides to save him, she jumps in with no hesitation, whereas she would have struggled to make this decision before tanjiro nudged her in the right direction
that is what kny and tanjiro's character are about specifically; it's about how giving is always rewarding, how kindness is strength, how having a bleeding, loving and tender heart will better you, others and the whole world. caring for others and being cared for is the solution
