Haven't been active in a while, but the hype around Arms character DLC has had me lurking all over and watching endless speculation videos around it. June is still so faraway though, so I wanted to write a nice long post on what character we could get.
First thing is that I don't believe we'll be getting a mixed character in the same fashion as Hero or Bowser Jr. It makes more sense to me for them to pick one character to represent Arms in smash than to try to shove multiple in there and risk either the characters animating and feeling out of character or spending unnecessary cost developing animations for multiple different skins.
Second thing is that I don't think Assist Trophies or Spirits will factor into the decision, assuming they were added into the base game. The logic here being that decisions being made for the second fighters pass likely took place after the base game was completed, so if Nintendo/Sakurai wants a specific fighter available they won't stop themselves just because the character is already in in a different form. Especially for Spirits since having a png of the character in the game shouldn't impact new content.
Last thing is that character popularity is something I don't find to be a reliable metric outside of certain circumstances (more on that below), and I think most people's take on who the most popular is depends on when their awareness of this game started.
From pre-release I would argue Twintelle and Min Min were the most popular following the hype of their announcement. Twintelle in particular was noted by devs following her reveal as being received with lots of love and them being pleased. However, after the game came out I recall the popularity of certain characters rising significantly (Ribbon Girl, Ninjara and Kid Cobra in particular), and I believe across different patches the devs shared use-rate and win-rate of the different characters. And now since several youtubers who were initially hyped about Twintelle and Min Min pre-release are jumping at them as likely, we see people who just now are getting into arms suddenly leap at these characters as "most popular".
So, what I expect to have been the obvious choices for the rep:
Min Min
For biased and unbiased reasons I think she's by far the most likely. The main unbiased reason ultimately comes down to devs and the party crash bash.
Min Min being one of the favorites of Mr. Yabuki and other Arms devs tells me she likely came up as an option during the conversation of what Arms rep to add (and how). I think that in itself gives her a boost.
Then you begin to think about the timing of the last party crash bash. It feels too convenient to me that in mid-2019, months before we learn there's more DLC packs, we get the "final" party crash bash as a tournament to decide who is the Arms champion among all the characters. While I don't think this in isolation would decide the character, I highly doubt that it wasn't intentionally planned as a way to gauge popularity among active players in a way that's highly visible and easy to push and reference in future Arms content.
As for my biased opinion, I think she also has the strength of being able to mechanically adapt to Smash better than other characters while still representing core arms concepts. Her kicks adding to possibilities for aerials and tilts while being in character is a freebie. Her dragon arm giving you charge indefinitely until you get knocked down could work very easily in smash as a way to integrate the mechanic of charging arms in a visual and intuitive way for Smash. And then you have the fact her default arms like the dragon translate well to being a special move that further focuses the character into long-range focus and spacing, which is apt for an Arms rep.
Spring Man
Probably not necessary to explain why I think he's likely. The main thing that feels off to me about Spring Man as a rep is that it would be kind of lame to tease and arms rep like they did and they just toss in Spring Man after giving everyone else hope. That said, it feels like it would be a very "Nintendo" thing to do and I don't put it past them.
Everything else positive applies to him. Even though not the "champion" of Arms by any means, he still seems fairly beloved by the Arms devs, is super recognizable, and has character traits that make him work well as an Arms rep.
Aaaaand, that's it. I think it's one of these two.
I could see a universe where they go Ribbon Girl since Spring Man is already an AT, but it doesn't seem as likely as the above.
Twintelle is "popular", but as mentioned I don't think she particularly wins in any actual metrics we know of. Devs know people liked her when she was revealed, but I don't know to what degree that lived on following release. The title of the bash between her and Min Min implies she was considered a fan-favorite (Fight over Fans), but... Min Min won, so it would be weird imo to go for Twintelle instead.
Ninjara is super popular.... but his mechanics wouldn't make sense in Smash for an arms rep.
Max Brass is probably one of the more likely ones, but I don't think he has as much recognizability as Min Min or Spring Man. Unless he gets in smash, then he'd work perfectly as a rep of the Arms League.
Dr. Coyle is a super cool choice... but I don't think her flying around and going invisible particularly represent Arms. I do think Hedlock would be an excellent stage hazard or final smash for an Arms rep and their stage though.
The rest are not very noteworthy imo. Helix and Lola Pop would be mechanically super fun but would be niche choices. Misango has interesting lore potential, but his core mechanic adds too much complexity that doesn't necessarily represent "Arms". As for Biff, I wouldn't consider him a fighter.
Lastly, I don't think we'll get an Arms 2 rep. I think it will be one of the characters shown during the last direct, doing anything else would be strange. If there is indeed an Arms 2, and I think there is, I think it's more likely that they'll add Min Min/Spring Man with a new render and possibly new movement or character features that will later make sense once Arms 2 gets shown. Kind of like how Luigi was shooting plungers in the Simon trailer and smash before we even knew this was a Luigi's Mansion 3 mechanic. Personally, I would love to see Min Min revealed as the standing champion in Arms 2 that the player has to defeat, and to have the belt as part of a new design for her, with her classic look as an alt.