ooc: Shakuyaku and Kagan
OKAY, CONTEXT: Dangaishuu are a group of monks who are "special ability users"/exorcists who oppose evil demons who attack humans. They started experimenting on spiritually strong humans to create elite super soldiers, trying to turn humans into demons because demons were that much stronger spiritually. The problem is that they used peasants (one of them being Shakuyaku) in these experiments as well, and this was not kosher in the least, so they end up pretty quickly as the series' main antagonists. And the reasons just keep on piling up.
Shakugan's introduction is as this massive rock demon who's going around killing people. We're told that she's one of the peasants experimented on and is a human who can use spiritual strength to solidify their body (which I presume means "transform into a demon"). She turns back into a human after Jinka crushes her to bits (literally) and doesn't remember anything before the "transformation ceremony", but regains her memories when she sees a monk transform himself and from that point, she is able to freely transform (at least) her arms.
It's not until later, when Kagan takes over Shaku's body for the first time and reveals himself, that it becomes known that the Dangaishuu's experiments involve taking demons and putting them in humans. In fact, it's actually revealed that Kagan was the one who transformed from the shock of the experiment and went on a rampage, although Shakuyaku also claims responsibility as not being able to control him at the time. However, unlike all the other transformed humans, Kagan's soul still exists because Shaku didn't try to completely dominate/devour him, and this results in a surprisingly harmonious relationship. For this reason they can battle effectively even against transformed humans with more spiritual power. (However, since his reveal, their cooperation in a fight is really only shown once.)
LONG STORY SHORT, WHAT DOES THIS MEAN FOR RP THEN?
Sprinklings of headcanon ahead.
Okay, so both of them will refer to themselves as Shakugan unless they're talking about the other. People generally refer to them in the same way: as Shakugan or the name of whoever's in control (although the group generally refers to only Kagan by his name, while calling Shakuyaku "Shakugan" or just "Shaku"; Kagan's really the only one we see referring to Shakuyaku by name).
In canon, Kagan only "appears" about five times. I assume most of the time he leaves Shaku's body to herself out of respect for the fact that it was originally hers and, well, because she's nice to him and there's no real need to.
In terms of voice: I'll be honest with you, until I started trying to read the Japanese raws, it was really hard to tell the two apart. Shaku is a bit more reserved and polite given her humble upbringing while Kagan, from what little we see of him, is more prone to ... being louder and more "'sup bro?". If I have to sum it up, the difference between them is almost like :) and :E.
I'm trying to draw as much from the Japanese raws as I can, but for once, a manga that's set in the the Sengoku period seems to follow anarchic Japanese speech patterns. Shakuyaku tends to speak with -su on the end of her sentences, usually associated with a lisp, but she also refers to herself as "watasu" which I've never seen before so I'm thinking it's more to do with the fact she's a peasant. Kagan, on the other hand, refers to himself as "ware" and from what I've gathered speaks with a lot of thees and thous. I'm not particularly keen on RPing either of these. :( And the scanlation team has them all speaking normally, sooo ...
I'LL TRY TO MAKE IT AS OBVIOUS AS I CAN OTL
SO, FIGHTING.
Shakuyaku will still generally be in control. Since it's been mentioned that she was experimented on as a "spiritually strong" person, I'm assuming it's something like "spirit reservoirs" in Kekkaishi and that she's basically got a lot of raw power/strength.
When it comes to using actual techniques though or requiring a bit more finesse, she seems to hand over to Kagan: the only times Shakugan's legs are also transformed seem to be when Kagan's in control. They're still shown to switch back and forth kinda randomly in the only fight we actually seen them cooperating in though, so I might have to wing it a little.
ALSO: SPOILERS. If you're canon-familiar: [ I'm playing it that they've "recovered" or been forcefully awoken from being a boulder for camp, so they ... don't really count as dead, I think? 8( They'll remember being stabbed, but they probably won't have any physical scar from it or anything. ]