This happens not without reason from her side - to be on point: Always when William was falling back into his "old-days-massive-jerk" behaviour.
Miru is depicted as a self confident strong woman, that don't take shit from anyone. She has her own mind, can distinguish between right and wrong and is for sure not a devote trophy girl for William that will brain dead ignore a misstep from him if he goes to far.
The reaction from her when she gets the knowledge about the "Monica-kitchen table-creampie" incident makes this clear. Or kicking William into the pool after his low blow towards Katie, or pushing William under water after verbaly rubbing salt into the wounds from Monica at the pool.
The expression "She is my better half" is no accident.
Her complete behaviour is totally comprehensible and understandable, she does the things that all girlfriends should do if the partner starts to be a jerk without reasons to others. And one thing is for sure: William was in the most cases not the innocent victim on the recieving end...