For the Ruined Home Temmie: I nicknamed them "Sign" since a sign is meant to give information on those who read it. And from what I've seen, They've been treated by the others as a weak link with minimal use. As they leave them behind in the Ruthless path when the Human decides to agressively approach. (I should now mention again that this contains spoilers for the Starlight Isles. Don't go further if you wish to have a blind experience of that area. I'll give you 26 lines to go back.)

I think that was 26 lines.

I also consider Sign to be treated as a Weak Link with a use because they're seen all throughout Ruined Home for a few purposes.

1°: to be a parallel to flowey following you in UNDERTALE.

2°: to keep watch over you.

3°: to keep the other Temmies hidden from not only the human. But Us too.

I mean think about it. We hear that they've been acting as a lookout in Ruined Home. But they've clearly known about us from before we met them in Ruined Home. Because they basically already planned an entrance by the time we get to them. And in Ruthless they even habe a plan to try to get you to join them, as the leader says something about "things not going as planned" if I remember correctly. And to finish off, I wouldn't think they're treated like that of it weren't for the fact that at least to my knowledge, they never can be seen following you again after you encounter the other Temmies. (I haven't played much, but from the times I saw the other Temmies Following me. I never saw Sign's Sillouette EVER. And it's not like they changed. I testes that in the Count's Base and the ferris wheel and it was always the same one, leaving me to beleive it's always the same Temmie for each time you get followed.) I haven't even seen Sign PERIOD After that encounter.

Now for the one with the Sleepy Hat, I'll call them Appeal. Since I think Sleepy is too normal for what we see of them, and They do hide in a few attractions.

For the other one we see that isn't the leader, I'm still cooking up a name for them. Since the only place I found them following me was at the Count's Base, and maybe the Vigilante Part of Starlight Isles. Though I'm thinking something like "Bugger" since they seem to be trying to annoy the leader, even if not intentional.

As for the Leader themmselves? I have none. I was thinking maybe "Top" or "Head" or perhaps "Headline", the first two because they seem to be the Leader of the Temmies. So Top for Top of the bunch, or Head for Head of the bunch, but "Headline" is because I see a bit of a similarity between them and the Story Teller from the opening scene of the game. If it was them it could make sense considering we speculate that Flowey is swapped with Temmie, but not the "Real one" as Team Switched has put. This along with the facts that Flowey is a re-ecarnation of Asriel, who switches with Monster Kid. And that Flowey when speaking to the Leader calls them by the Nickname "Plushie" could explain them being to one to tell the story if they were to be the the Story Teller, and also Monster Kid. Also Flowey calling the Leader "Plushie" is my other reason to not give the leader a Nickname. I decided to just use Flowey's Nickname for them!