This was one of the cards I was most hyped to see out of this set, and honestly, I'm disappointed. Stealing all 100/100 counters on the field is a neat mechanic, but that's it; it's just neat. It sucks away all of YOUR counters, so all the work you'd put in to pumping your Second or Third Daughter just got stolen, meaning your Third Daughter can't even attack unless you get more counters on her. 100/100 counters as a general mechanic aren't seen nearly enough in most decks for this to get any real advantage from stealing counters from the opponent, either. This can get REAL big against a Force deck, but those aren't exactly common. The biggest issue is that you're putting all of your eggs into one basket, so any removal spell that gets rid of Yashahime after she vacuums up your counters means ALL your counters are just gone.
Her second ability kind of makes up for that, and in the long run it will double the number of counters you initially had, but the long game doesn't happen too often, especially since she'll be a pretty big body that should be smashing face.
This guy is the first in too long of a line built to support Yashahime. 700/700 for 2 will is above what is allowed, meaning he's the strongest 2 drop you can possibly have, with the trade off of pumping one of your opponent's resonators 200/200 when he enters. The idea is to play him, pump something of the opponent's, then play Yashahime the next turn to steal the counters. That's gimmicky and really doesn't get you anywhere, given the issues I already brought up with Yashahime herself. You get this guy for full value if you manage to play him when the opponent's field is empty, but even then you'd just be getting a 700/700 vanilla which doesn't mean much.
Here's something a little more plausible in the "Playable Yashahime support" category, albeit a weak entry. The idea with Yashamaru is to use the cards that put 100/100 counters on your opponent's resonators, which opens the doors for him to be able to flat-out kill them. It's essentially a less consistent version of the Seijuro + Urashima combo that relies on pumping your opponent's cards. If Yashamaru AND a counter-adder were playable at instant speed then maybe, but even then you'd be killing a 100/100 counter that Yashahime would want to steal. This guy is a cute, niche side option against Force and against decks that like using their Nameless Mist, and really nothing more.
The last Yashahime support card revealed in this spoiler, it's likely the most playable of the set. An instant speed 2-mana card that forces the opponent to banish a resonator. Pretty neat, right? Aside from giving every other J/resonator they control a 100/100 counter, although that at least has its work-arounds. Against something like Fiethsing Turbo, which is the most prominent meta force at the moment, you'd certainly side this out. Against decks that only stick to 1 or 2 resonators, though, this thing is nasty, especially since it gets around Barrier.
I really don't like this whole "support Yashahime" theme they did, since Vampires themselves weren't fleshed out to begin with. They didn't need a sub-theme, and a bad one at that. I'd have much preferred them to give us a couple more cards that give your own Vampires 100/100 counters, or interact with your counters in some manner. Or at least made Yashahime better and more synergistic with the rest of the deck. Oh well, at least there's still hope in the Fourth Daughter.
This card is... meh. I REALLY wanted it to be good, but I just don't see it. It's searchable with Gill Lapis, who can Shift to put it into play, but that's really it. Hard casting a 6-drop is no small feat, and the amount of reanimation effects are few and far between, plus there are just better reanimation targets. This guy is really strong against decks that go wide, but the only relevant deck that does that is Fiethsing Turbo. RFGing Guins, Rukh Eggs, Messenger Familiars, Tamas, and all the 1-drop weenie turbo fuel they use is an amazing effect, but alas, it won't usually happen. Fiethsing's tokens are 0-cost resonators, so more often than not, this thing will hit the one or two of them that are lurking on the field. Cheesing him into the field with Gill means you hit one or two tokens, swing for 12 in the air (or maybe trade with a Gwiber), then he gets removed from the game with Shift Gill's effect. At that point you're still just better off with Behemoth for the guaranteed 1500 damage.