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Mettaton in Deltarune is a much more depressed and repressed version of himself, compared to how he was in Undertale. Talking about how much of a nobody he is, and never seeing him come out of his house... However, giving Tenna to him to brighten his mood I think is genuinely doing wonders for him. Tenna I think is what's responsible for MTT truly finding his passion in the art of show business, as well as cracking his egg along the way, and gets him to start coming out of his shell more, or at least trying to.

Some miscellaneous thoughts here:

Tenna, unable to really communicate the normal way (y'know, talking), he instead opts to sentence mix whatever TV programs or movies MTT is watching. Chopping up bits and pieces of dialogue to form as coherent of sentences as he can muster. MTT is both surprised and very intrigued when his television suddenly starts stuttering and continuously rewinding the phrase 'What's your name?' from one of the characters in his show.

It's Tenna's idea for the most part to get MTT to try again with the whole 'making a body' thing. Seeing as how the NEO body in the Cyber World was a scrapped design MTT had for himself, him and Tenna start a little smaller and more reasonable, leading to the classic box design as opposed to his EX form. Now, neither of them know how to build a robot, but.. it's a better start than before.

Tenna is MTT's personal hype man. Sometimes leaving the house to go get groceries is extremely difficult, but he's always there for him to make the job feel easier. DR!MTT is effectively a hikikomori for the looks of it, so having somebody so bright and positive like Tenna in his life now is bound to have an impact on him.

This impacts Tenna too, of course: having somebody so captivated by him again is doing wonders. MTT could not appreciate Tenna's new presence in his life more. They are dependent on each other, but healthily.