Matsuo the Demonslayer is one of the most popular and most-referenced pop culture characters in the Metroplex.
TV
Matsuo the Demonslayer is included among the common TV programs.
Seeing Matsuo on TV gives you a chance to win the replica sword.
Skills
Effects
Channeling Matsuo
Training Hook
Techniques
There are four techniques (that we know of) that you can learn from fighting with the Channeling Matsuo effect active: Parry, Swordplay, Leaping Dodge, and Power Strike.
Other techniques that relate to Matsuo:
Demonslaying Blade
Project Rag Man
Ragged Tendrils
Items
These items mention Matsuo or one of his opponents (see below) somehow:
Blade of the Snows
Blood Rose Crown
defense knife
demon face
eternal sparkler
fanmod Idoru
Matsuo finale
Rag Man's Cape
replica sword
shark hide baton
skeletal staff
solid Rag Man hologram
While nowhere in the description or known associated content does the dancing hut figure explicitly mention (Matsuo foe) Baba Yaga, it's pretty clearly her hut.
Similarly, the goblet of bones/bloody goblet/overfull goblet don't mention Vlad Tepes but might relate to him (Zack does mention it).
Opponents
From TV we know the names of quite a few of Matsuo's foes.
Baba Yaga
Chaos
his own dark reflection
the Cackling Skull
the Dragonmaster
the Kappa Lord
the Lady of Snows
the Lord of Blood
the Master of the Deathtouch
the Mountain Oni
the Nameless Race
the One-Eyed Wanderer
the Ooze Beast
the Queen of Death (her minions were possibly known to wield bony swords)
the Rag Man
the River God
the Serpentine Dragon
the Spider Woman
the Stone Beast
the Three-Eyed King
the White Worm
the Worldsoul
the Zombie Lord
Vlad Tepes
<one of the above> and <x>'s more powerful ultimate form.
The Towering Figure encounter mentions another Matsuo opponent: the Awakened Dragon God.
Some of these opponents can be encountered in-game:
Black Stone Beast
Rag Man
Rag Man Hologram (opponent)
The Rag Man's More Powerful Ultimate Form
Snow Maiden
Many opponents not directly mentioned on TV count as demonic for purposes of the Demonslaying blade technique — see Opponents Testing for a full list. According to Podcast 53, (if Kinak remembers correctly) the list of opponents is a "white list" rather than a "black list", so all opponents not specifically certified as OK are assumed to be demons.
Encounters
These encounters include some significant result related to Matsuo:
Hacking
Some hacking results tie in to Matsuo:
shared files
post archive university net
NPC Mentions
Some encounters or contacts mention Matsuo seemingly in passing:
Dreaming of a White Christmas
Trio of Guards
05cleaner
Zack
Avatar
Kinak confirms that there is currently no Matsuo avatar in the game.
Podcasts
#26:
Q: Have we yet met any of Matsuo the Demon Slayer's enemies, other than the Rag Man/Ultimate Rag Man? If not, will that be part of future content?
A: Keep in mind for a moment that Matsuo is not real, even in game. But it would not surprise me to run into some stuff for a variety of reasons.
#36:
Q: Why is Matsuo so awesome?
A: Classic campy samurai that really makes me happy.
Q: Are we missing anything particularly neat related to Channeling Matsuo?
A: Oh yeah.
#51:
Q: And for some other long term stuff… Of all the mysteries we've not yet managed to solve since the game started, can you tell us anything about your favorite?
A: There is a, obviously quite a few refereces to Matsuo the Demonslayer. There's a bit more to that than just callbacks to a cool show, and I think people will be very pleased when they find that.
(The Demonslaying Blade technique and Demon Slayer skills were discovered after these podcasts aired.)
#53:
Q: How much more cool Matsuo stuff is still out there?
A: There's definitely a fair bit, I mean at least one more decent-sized chunk of goodness. Actually, I put in an art request for a bunch of Zaibatsu stuff, I don't remember if there is any Matsuo in there.
(Kinak confirmed on Podcast 54 that the chunk of goodness was Fire Dancer.)