Reminder that;
>Ramb saw Kris opening the dark fountain
>Ramb calls Kris out for not actually wanting to have fun, removing this as a motivation for him opening the fountain
>Ramb figured out that Kris was motivated more by Toriel, saying “Mums the word, ain’t it?”
>Tenna’s goal is to bring back the good old days and get Kris to smile and enjoy himself again
>Dialogue from the Mantle games suggests Tenna made a “deal” with Kris
>When Tenna takes calls in between the boards he refers to needing “air holes” in something that was found, and talking about someone “turning over” while they sleep
>When we see her Toriel is asleep in a gachapon ball, which would need to be ventilated for her to breathe
>Kris is not actually impressed with Tenna. Ralsei and Susie imply he is not having a good time, and Kris does the most damage against Tenna in the fight.
>When the Roaring Knight arrives they immediately try to kidnap Toriel, who is conveniently next to the house’s front door relative to their location in the dark world thanks to Tenna.
>Fighting the Knight distracts them from kidnapping Toriel long enough to get interrupted by Undyne, who is captured instead.

So basically, Tenna was Kris’s useful idiot who agreed to find and capture Toriel in exchange for getting a chance to relive the past with Kris. Kris wanted Toriel to be captured by the Knight, but Susie fighting back foils this plot and Undyne gets captured instead.