You can find Biscuit's kit on the official NIKKE Twitter.
A review for Biscuit will be released once we will actually be able to see her in action and test her ourselves. The first impressions are based solely on analyzing her skills.
We also enabled her skills on her profile, so you can check them there.
Weapon: Rocket Launcher
Squad: Happy Zoo
Skill 1 - Happy Puppy
■ Activates at the end of Full Burst. Affects all Attacker allies.
Critical Rate of normal attack ▲ 5.77% for 10 sec.
Constantly recovers 1.53% of caster's Maximum HP every 1 sec for 10 sec.
Biscuit's S1 skill is a decent Critical Rate buff, but only for normal attacks - this means it won't affect skills or burst from her attacker allies. The healing part of the skill is honestly a bit underwhelming as a 15% max HP heal over 10 seconds doesn't seem like much.
Also, what's interesting about the skill is that it activates after Full Burst ends. This is pretty unique as skills usually activate when entering the Full Burst to maximize its power.
Skill 2 - Tug of War
■ Activates when a Defender ally's HP falls below 50%. Affects the target Defender ally.
Invincible for 5 sec. Activates 2 time(s) per battle.
Recovers 23.26% of caster's Max HP. Activates 2 time(s) per battle.
Guess after trying to make AOE Tanks work under a high power deficit and failing miserably the devs are really going deep into the cheat death mechanics. First, it was Makima, who could cheat death once per battle, but now they added Biscuit, who can give the ability to ANY Defender and it also triggers twice per battle too (and if you pair Makima with Biscuit, you will get 3 cheat deaths).
Will Biscuit finally make AOE Tanks viable in PVE? Yes, but only some of them.
Because Biscuit's Burst cooldown is set at 40 seconds, it means you need to run a second Burst II character with her. Thankfully, two AOE Taunters belonging to the Burst II category have 20s Bursts - Sin and Makima.
Now, where you can use the combination to your advantage? Against some bosses (especially Train) and in PVP where you can basically create a wall that the enemy will have a hard time going through.
As for other AOE tanks:
If you will want to pair Biscuit with Ludmilla, you will have to forget running Liter or a Healer in your Burst I spot,
if you will want to pair Biscuit with Xmas Ruppe it will force you into a weird 2-2-1 formation that will be too defensive to kill anything,
Noah has 40s Burst too, so running both her and Biscuit will be a bit tricky.
Overall for PVP, running 40s Burst II characters isn't an issue as in most cases the battle is over before the second Full Burst, so this makes Biscuit much more flexible there.
Burst Skill - Walk Training
Cooldown - 40 seconds
■ Affects 2 ally unit(s) whose cover has been destroyed.
Rebuild cover with 93.6% HP.
■ Affects all Supporter allies.
ATK ▲ 43.08% for 10 sec.
Restores 55.44% of attack damage as HP for 10 sec.
We already have two characters that can heal cover (Liter and Cocoa), but Biscuit is the first character that can rebuild a cover that already has been destroyed. And at the max skill level, the cover will be rebuilt with nearly full HP too (it's around 50% at level 1).
This ability will become really useful on bosses as it will allow your allies to survive for longer - this further cements that Biscuit is made for Union Raid and Interception.
The second part of the skill is terrible though because of the restriction - it only affects Supporter allies. If the skill affected all allies, it would be perfect and it would fix several issues with Biscuit's kit, but it doesn't - making it basically useless because:
Supporters don't really need ATK (and healing based on ATK) as their base damage is meh (in most cases),
Pepper is the character that will be really happy from pairing her with Biscuit as she's the best damage dealer in the Burst I category and with Biscuit, her damage will massively increase.
You will run at most 1 Supporter character with Biscuit and that will be your Burst 1 spot - so the buff will affect only that character and Biscuit.
Biscuit seems like a niche character that will shine against some bosses (especially in Union Raid where you need to prepare 3 teams), and that will be really annoying in PVP (in stall comps). Being a Rocket Launcher wielder, she will also have at least decent burst energy generation which is really important in those modes.
You won't really use her in your campaign progression as she's too restrictive team-wise and you will lose more than you gain - for example, Rupee as Attack buffer or Centi as Shielder and Burst generator will help your team more than Biscuit.