Disclaimer
Helm's kit was just revealed on the official KR community and you can find the original here.
Please be aware that we won't be rating her and this means she won't appear in the tier list - you will find reasons why we decided to do that in the post. Also, we already added Helm to the website and you can check her profile here.
Helm

Rarity: SSR
Element: Water
Weapon: Sniper Rifle
Burst type: Burst III
Class: Attacker
Manufacturer: Elysion
Skills
The value shown below matches the level 10 skills of the character.
Basic Attack
■ Affects target enemy.
Deals 69.04% ATK as damage.
Charge Time: 1 sec.
Full Charge Damage: 250% of damage.
Skill 1 - Frontline Command - Passive
■ Activates when the last bullet hits the target. Affects all allies.
Critical Rate of normal attack ▲ 14.64% for 5 seconds
Skill 2 - Fire Away - Passive
■ Affects all allies.
Damage to interruption part ▲ 3.08% permanently.
■ Activates when entering Full Burst. Affects all allies.
ATK damage ▲ 11.85% for 10 sec.
Burst Skill - Aegis Cannon - 40 sec.
■ Affects the enemy with the highest ATK.
Deals 1237.5% of ATK as damage.
■ Affects all allies.
Recovers 54.45% of ATK damage for 10 sec.
Review
Helm is a Burst III Attacker who shines in boss fights and specializes in supporting her allies.
Her S1 boosts the Critical Rate of all allies when she fully empties her magazine which is easy to do since her weapon of choice is a Sniper Rifle (but the buff only works for normal attacks, not skills), while her S2 boosts the ATK of her allies. The second part of her S2 permanently increases the amount of damage Helm and her allies do to the parts of the bosses appearing in the red circles during fights with them - which means you can interrupt their skills faster. Oh, and she also provides Lifesteal to her allies, which is perfect for boss modes as it allows you to not use a dedicated healer to push the score even further.
Helm's biggest issue is that her skills either focus on buffing allies or dealing damage to one enemy, which means that in non-boss stages, her damage output will be way below Harran and other characters specializing in AOE damage. Single target damage-wise, she's a bit weaker than Alice, but she makes up for it with all the buffs she provides to the team.
Overall, Helm is a decent buffer and damage dealer hybrid who shines in Coop, Interception, and Boss stages, but struggles in stages where are a lot of enemies.
Rating
As we mentioned earlier, Helm won't appear in the tier list and there won't be a rating visible in her profile, because on the test server she was bugged.
Her S2 instead of giving ATK to allies was actually lowering it by the same value. And since in NIKKE, buffs affecting allies, also affect the character itself, she was basically making herself weaker all the time. We only realized it after we ran all our tests, so we wasted a lot of time and had to redo them.
Here's the bug in action:

We reported the bug to the developers, but the test server wasn't updated yet with the fix, so we were not able to test re-test Helm again and just adjusted our findings for the purpose of this review.
Still, even with the bug fixed, Helm would land in the A rating on our scale. Her damage in non-boss stages is comparable to Alice but while Alice is very selfish, Helm brings nice buffs for the whole team. Still, when it comes to Snipers, Harran remains the Queen and Helm won't change it as she deals less damage in both boss and non-boss stages.
Our tests
While we would love to show you the max level/core up/skills tests we spend hours doing, they can't be shared, because Helm's kit was updated between the time we did them and now, and because of the bug mentioned earlier.
So we had little time to retest Helm and only were able to do a few tests in a sub-optimal environment (level 80 characters with level 1 skills). So to avoid spreading misinformation we won't share any screenshots, but we will retest Helm in the coming days, and only then she will appear on the tier list.
Should you pull on her banner?
That's the million-dollar question.
Those who like what Helm has to offer actually won't lose anything if they dump all their savings into the banner even if they won't pull her, simply because they will receive Golden Mileage Tickets anyway - they can use them to buy characters that will appear on future Special Banners. And if Helm surprises them, it's a nice bonus.
Those who don't like Helm and want to save for the next banner should do so.
And those that are still rerolling, I think getting Scarlet or Harran is the superior choice as those two are just performing much better than any other damage dealer in the game.