No one can stop you from having a name

Of course there is the question of who is Mustard. My immediate thought was of course Ralsei. He is Kris' closest friend in the dark world. He takes the form of their brother following his absence from their life. But while Thai(Kris)'s relationship with The Imaginary Friend is integral to the story, so is Sadie(Toriel)'s. I'd want to embrace the moments of the play in which a mother unabashedly explores love and sex, as well as the broader themes of divorce and detached fatherhood.

This has me thinking Tenna immediately. Have people fucked around with Toriel/Tenna before? This ties everything together wonderfully. Someone(thing) that Kris and their family once shared, a person(thing) that Kris was very attached to as a child and sort of represents the "before" (see, well, Tenna's whole thing with Kris in ch3). While Kris(Thai) clings to the past, they are quickly becoming tired of Tenna(Mustard)'s games. They lash out. They become violent and unpredictable. Tenna, of course, is also desperate for things to remain the same, as he is in the game. It makes sense to me.