Chronologically:
Mato started her year at school
Mato met Yomi
Mato and Yomi join different club
bond created
new semester/year/whatever
they are in different class
Mato got new friend (Yuu) and became more distance with Yomi
Yomi jealous and ran away from home
Yomi randomly became a host for Dead Master
Mato felt something and randomly became the host for BRS
Fight
MatoXBRS extracted Yomi from Dead Master
hug and friend forever
Yuu (who shall became Strength if there's a sequel of some sort)'s eyes sparkle with jealousy when she saw Mato and Yomi together
the end


scene at the start with BRS fighting with Dragon Slayer have nothing to do with the whole thing.