What we know about the leakers:
The images and videos are all real.
There are 7 named leakers as part of this leak:
1) All-Star. Already shown to troll. Leaks on 4chan's /v/. An associate of MFLeaker, perhaps.
-Claims Dixie, Mewtwo, and someone else are in.
2) Motherfucking Leaker: Also shown to troll. Leaks on /v/ as well.
-Possibly claims Dixie, Mewtwo, Ridley, Mach Rider, and Impa are in, though this is disputed.
3) NessInEngland: Not shown to troll. Hasn't reported any false information, has speculated, but has always indicated it is speculation. Claims that no one knows who the other characters or DLC is, iirc. Leaks to Gamefaqs.
4) MasterLinkX: Shown to and admits to lying, for whatever reason. Leaks to Smashboards. None of his claims are to be trusted.
5) VaanRose: Source of the text leaks. No reason to believe he trolls. Claims All-Star and MFLeaker are lying. Leaks to Smashboards.
6) NeoZero: No reason to believe he trolls. Mentions that a Nintendo Representative said that there's more characters in the game, including surprises. Vouches for VaanRose.
7) Wubudubadubadattrue: Leaked the videos today. Claims/speculates there's more characters in the game, but makes no mention of who, and is likely just speculating. Leaks to Reddit. Name is notably a Rhythym Heaven reference
Other entities:
ESRB guy: The person from the ESRB and the source of all these leaks. Only MFLeaker or Wubuduba could be this person.
Unnamed group: There's an unnamed group that the leak's been passed around in, who get it from the ESRB Leaker. Many of the above leakers are either in or have sources in that group
Sal Romano: From an entirely seperate leak, the Gematsu Leak. Leaked many newcomers, incorrectly leaked Chrom, and mentioned Chorus Men and Shulk would be in, Lucas might get cut, and there could be DLC. He is not at all connected to this leak, and does not know anything outside of what's been mentioned to him. It's worth noting he reports his info from a source at Namco Bandai, not at the ESRB.