Source
Link to the official patch notes: check here.
Summary
New banner - Jackal
New banner - Viper
New character - Quency
New character - Sin
New character - Guilty
New story chapters added - 19 and 20
New game mode - Liberation
New Lost Sector added - 12, 13 and X3
New event - D-Outsiders
New coop event - Nihilister
New features and improvements
Woah, that was a lot.
New characters
Five characters will be added with this patch - 2 will be available on new banners that will start after the maintenance and 3 will be available from a new game mode - Liberation.
Jackal

Jackal's banner will start on the 12th of January and she isn't a limited character - you can purchase her using Golden Milestone tickets. After the banner ends, she will be added to Shards and Generic Pool.
Some information about her:
Burst: 1
Class: Defender
Rarity: SSR
Weapon: Rocket Launcher
If you want to check her kit, head to her profile page - we already added her skills there.
Viper

Viper's banner will start on the 12th of January and she isn't a limited character - you can purchase her using Golden Milestone tickets. After the banner ends, she will be added to Shards and Generic Pool.
Some information about her:
Burst: 2
Class: Attacker
Rarity: SSR
Weapon: Shotgun
If you want to check her kit, head to her profile page - we already added her skills there.
Quency

Quency is a free SSR character everyone will be able to obtain from the Liberation Mode that will be released with the same patch. We don't know how long it will take to obtain a character from that mode and a guide will be prepared once we know more details.
Some information about her:
Burst: 2
Class: Supporter
Rarity: SSR
Weapon: SMG
If you want to check her kit, head to her profile page - we already added her skills there.
Sin

Sin is a free SSR character everyone will be able to obtain from the Liberation Mode that will be released with the same patch. We don't know how long it will take to obtain a character from that mode and a guide will be prepared once we know more details.
Some information about her:
Burst: 2
Class: Defender
Rarity: SSR
Weapon: AR
If you want to check her kit, head to her profile page - we already added her skills there.
Guilty

Guilty is a free SSR character everyone will be able to obtain from the Liberation Mode that will be released with the same patch. We don't know how long it will take to obtain a character from that mode and a guide will be prepared once we know more details.
Some information about her:
Burst: 2
Class: Attacker
Rarity: SSR
Weapon: Shotgun
If you want to check her kit, head to her profile page - we already added her skills there.
New mode - Liberation

The Liberation mode was announced recently and we don't know much about it currently, Only one thing is certain - you will be able to slowly farm three characters there (Sin, Guilty, and Quency).
Those three characters won't be obtainable in any other way, just from the mode, and also you can only focus on one of them at a time.
New event - D-Outsiders

Once the Brand New Year event will end, we will get a new one - D-Outsiders! Follow the story of the Exotic squad and learn more about Crow, Viper and Jackal.
A guide for the event will be released once we know more about it.
New coop - Nihilister

In the next coop event, we will fight against one of the brand new bosses - Nihilister. The image above shows how does it look like.
The coop event will be held from 20.01 to 23.01.
New features and improvements
With this patch, we will get a lot of new features and improvements and you can find the list below:
Animation of all Nikkes’ Burst Skills have been added to the live wallpapers. Complete the three stages of a Nikke’s Limit Break to obtain her live wallpaper.
Three CGs have been added to Chapter Stages. A new CG can be found in each of the following stages: Chapter 14: Bait, Chapter 14: Face-To-Face, and Chapter 15: Shopaholic.
New album TuNGSTeN (Hiroyuki SAWANO, feat. Mizuki) added in Jukebox.
New chat function in Union (unavailable for minors).
New commodities in Arena Shop. New T7Manufacturerequipment will be available after player clears Stage 15-12, and new T8 Manufacturerequipment will be available after player clears Stage 18-33.
Rookie Arena victory mission added to daily and weekly missions.
New landscape mode added. To play the game in landscape mode, change the mode in Settings.
Adjusted the balance of various characters.
Enabled the Skill Reset function temporarily due to the big-scale character balance adjustment.
Skill Reset duration: From the end of Jan 12, 2023 maintenance to Jan 26, 2023 23:59:59 (UTC+9)
How to reset: Nikke → Character Information → Skill Uninstaller in Skill to reset the character’s skills.
Three Skill Uninstallers will be distributed through mail.
All skill development materials used before will be returned after the Skill Reset.