>>4
Temmie is a real X-factor in this case. She is an UNDERTALE character, so in that sense Toby would rather not emphasize her over the original cast, but as the same time she is a "dev" character, sometime with far greater insight into the rules and mechanisms of the game world than any other character would reasonably have. She knew of and could remember resets well enough to give you discounts at the Tem Shop after all.

From a self-contained standpoint, her character so far mirrors that of Berdly's, wanting to be recognized and respected but not knowing how to get that in a healthy way, so instead of building an obnoxious mask around being "smart"(which we the player know she is in some sense at least, she managed to design Temmie Armor) she spends her days plotting revenge. The Dark World is the perfect opportunity for her to both take this revenge(especially against Susie; Catti-Temmie team-up anyone?) and to show that she really is more than just the village idiot, and for Catti to get rid of one more obligation on her already overloaded plate with family that are too dumb to recognize or care for her needs.

Catti is very obviously loaded up to be Susie's final character test; too strong for her to overpower in the Light World, likely too much of a magic dork for her to fight in the Dark World, and too mature for her to intimidate, forcing Susie to confront her with the new people skills she has learned over the course of the story or perish.