The file in question is a character parameter titled "fighter_kind_brave". Looking at the expanded list of parameters viewable on Github, it's clear all the fighters are named this way, so we can assume Brave is indeed a character's name. But how does that lead us to Dragon Quest?
Protagonists in Dragon Quest games are generally referred to as heroes, especially in Japanese, where their default names are often literally "hero." As it happens, written in Japanese, the word 'hero' can also be read as 'brave.' Moreover, the two words share kanji. Not only that, Dragon Quest is a beloved JRPG series, and between a Japanese Switch release of Dragon Quest 11 and Dragon Quest Builders 2 (which launched just last month), it's ripe for some Smash-fueled buzz.
This connection makes more sense when you consider that several Smash characters have Japanese names in their parameters, including other downloadable characters. The Piranha Plant, for example, is listed under "fighter_kind_Packun", which comes from "Pakkun", the Japanese name for the Mario-hungry flower. Pokemon such as Ivysaur, Incineroar, and Greninja, as well as characters like Isabelle and Corrin, are also listed under their Japanese names.
Ok so since the secret's out, here's confirmation. Params in Ultimate confirm the code names of 2 future characters, "jack" and "brave".The labels for these characters:fighter_kind_jackfighter_kind_bravefighter_kind_packu (piranha plant) pic.twitter.com/rCRRg8KR3WJanuary 3, 2019
Joker's parameter, "fighter_kind_jack", is a little stranger. Written in katakana, Joker and Jack do share the character for "ji", albeit with a different modifier, but that's the only real link I could find between the two names. It's possible this is just a code name taken from Jack Frost, the winter sprite who is basically the face of the Shin Megami Tensei franchise which Persona branched off from, or maybe Joker's name was just flattened in translation. In any case, no other parameter file is a match for Joker, so Jack it is.
A Dragon Quest DLC character looks plausible, but who is it? The most popular candidate is Erdrick, the hero in Dragon Quest 3 and a pivotal figure in the series' lore. The hero of the original Dragon Quest, for example, is a descendant of Erdrick. As noted by dataminer BenArthur_7, a member of the Smash group Kurogane Hammer which originally uncovered the Brave parameter, the Japanese name for the hero in Dragon Quest 3 can be read as "brave man." (Incidentally, BenArthur_7 also said Kurogane Hammer first spotted Brave on December 5, but kept it under wraps "out of respect for the dev team." Evidently it was leaked this past week, so here we are, unable to get the cat back in the bag.)
There's more evidence that these parameter files correspond to DLC characters, too. I spoke to software engineer and Smash modder jam1garner, who contributed to the datamining, and they said the parameters for Piranha Plant and Joker were present in the 1.0 version of Smash Ultimate, while Brave was added in version 1.1. This lines up with director Masahiro Sakurai's previous statement that all of Smash Ultimate's DLC characters were determined ahead of time. It's possible future DLC characters will be uncovered this way, but as jam1garner put it, "it went public so Nintendo or whoever might start cleaning up behind themselves."
Here's how to unlock all the characters in Super Smash Bros. Ultimate.