Disclaimer
This isn't Modernia's review and we won't rate her, because it's way too soon, but after a few hours of testing we learned a few things about her and you should be aware of them before you pull.
About Modernia

Manufacturer: Pilgrim
Weapon: LMG
Element: Fire
Burst Type: 3
Class: Attacker
Skills
Let's go over her skills and how they work.
Normal attack
■ Affects target enemy.
Deals 7.71% ATK as damage.
Modernia's regular attack damage is actually the highest among the LMG users - while they on average deal 5.5% ATK per shot, Modernia deals 7.71%.
Also, do keep in mind that when Modernia is using her Burst skill, the normal attack damage is disabled. This doesn't look like a bug, but more like a balancing attempt, because without it Modernia would be beyond broken - more about it later.
S1 - High-Speed Evolution
■ Activates when normal attack hits. Affects the target(s).
Deals 3.05% of ATK as additional damage.
■ Activates when normal attack hits 200 time(s). Affects self
.Critical Damage ▲ 14.25%, stacks up to 5 time(s) and lasts for 10 sec.
Max Ammunition Capacity ▼ 5.04%, stacks up to 5 time(s) and lasts for 10 sec.
Modernia's S1 heavily affects her damage output - in two ways. First of all, every bullet she fires will deal not only deal the 7.71% damage from the normal attack but also the 3.05% as bonus damage. That's why while using her, you will see a lot of numbers on the screen.
The S1 bonus damage will also be active during her Burst, unlike the normal damage.
The second part of the skill increases her Critical Damage and decreases her max ammo capacity after she fires 200 shots. Not only this buff can stack up to 5 times (and you will get juicy 70%+ Crit DMG, but also Modernia gains the stacks while she is using her Burst, so she can quickly reach the max stack count. About the negative part of the buff - the ammo capacity - you don't really feel it that much.
S2 - Giant Leap
■ Affects all allies. Activates when entering Full Burst.
Hit Rate ▲ 8.56% for 15 sec.
■ Affects self. Activates when normal attack hits 200 time(s) during increasing Hit Rate status.
ATK ▲ 29.38% for 10 sec.
Modernia's S2 allows her to give a Hit Rate buff to all allies, and while the value isn't that big, it always helps. And the second part of the skill affects only Modernia, and it boosts her ATK by quite a lot, but she needs to be under the Hit Rate buff to activate it. So while normally, you would only get the ATK buff when another ally gives her that buff or when she enters Full Burst, there's a trick to activate the buff 100% time.
What's the trick?
You simply need to give Modernia the Assault Cube as the cube's passive increases her Hit Rate. Permanently. But since the buff appears on her buff list when you pause the game, whenever she fires 200 shots, she will instantly get the juicy ATK buff.
We don't know if this is a bug or intended behavior, but we can abuse it for now.
Burst - New world
Cooldown - 40s
■ Affects all allies.
Full Burst Time ▲ 5 sec.
■ Affects self.
Grants unlimited ammunition for 15 sec.
Destroy Mode:
Extending the line of sight and auto-aim at all enemies within fire range. Stage enemy will be recognized as a single enemy regardless of its interruption parts.
Deals 2.24% of ATK as damage for 15 sec.
Now that's an interesting burst skill.
The first part increases the Full Burst Time by 5 seconds for all allies. While till now we had characters that decreased the FBT (like Isabel), Modernia does the opposite and it translates to a big DPS increase for the whole team. Still, there are a few downsides for this, as it throws off the rotations because you're delaying Liter's Burst skill (this also affects other characters that want to burst as often as possible).
As for the second part of the skill, it transforms the aiming reticle into a big circle and Modernia hits everything that's inside of it which makes her a terrifying AOE DPS that can melt whole groups of enemies at the same time.
The only issue is that she deals the 2.24% ATK damage and the S1 additional damage during that time - the normal attack damage bonus isn't applied. This means that against a single target, her DPS actually goes down when she starts Bursting. While for boss fights that also spawn smaller enemies this isn't as big of an issue, against most solo bosses you can feel the difference in her damage output.
Still, then you can actually use Modernia in the last spot in 1-1-3 formation, so she won't ever burst. Her damage output then is actually decent, but we need to test it more.
Oh and also do keep in mind that while Bursting, the big circle treats the boss as one entity and Modernia won't hit multiple weak spots (like turrets) at the same time and just deal one instance of damage (and it will hit the part you aim at).
First impressions - summary
Here's a summary of what we noticed so far about Modernia:
Modernia's damage output in story stages where you fight waves of enemies often surpasses Harran,
Modernia's damage output in story stages where you fight a boss + waves of enemies is also really good, but we need to test it more,
Modernia's damage output against solo bosses still needs to be tested, but currently, we can call it good/very good depending on the boss you fight (on Chatterbox, for example, she is on par with Scarlet damage-wise). We also need to test the 1-1-3 formation more, but that will be possible after the reset,
Modernia's Full Burst Duration increase should allow you to build teams that don't focus on CDR that much and she might be the start of a meta shift from the Liter/Dolla combo,
Always use the Assault Cube on Modernia so she has an easier time activating her ATK self-buff (you can even keep it permanently on for the whole battle once you activate it once),
During Burst, Modernia's circle of death allows you to bypass the annoying Shield mobs that block your view (and shots) as she doesn't care about it and hits everything in her circle anyway,
Modernia's damage output is really reliant on skill-level upgrades (S1 -> Burst -> S2 leveling order, but try to keep them on the same level as all are important for her),
Modernia is still really good when she is fighting under a heavy power penalty so she's a great character to push the story (we've got a lot of reports from players that slotted her into the team without even investing in her heavily and she allowed them to push through the wall they were stuck on).
Should you pull Modernia?
Considering that she is the first Pilgrim banner you will have an easier time MLBing her (if luck is on your side) so she's a great character to pull, especially if you're lacking a good mobbing character (with Harran being the best choice till now for that role, followed by Scarlet). While Scarlet/Harran (and recently SW) are all top-tier Pilgrim damage dealers, without being lucky, MLBing them might take you months if not longer.
We should be able to test Modernia more in the coming days and then we will release her full review and add her ratings, but for now, we can say that she is really strong and probably the new best mobbing character in the game that is also decent against bosses.