Here's all characters in Call of Duty Mobile. Some of the characters are also known from other Call of Duty series as well.
I've listed the character into their own collection since some of them have other skins customization.
Call of Duty Characters: Battery
Real name: Erin "Battery" Baker.
Character appear in Call of Duty: Black Ops III, Black Ops 4 multiplayer modes.
All Call of Duty Mobile Battery skins:
Battery - Demolitions
Unlock in: Battery Solider Crate.
Battery - Sting
Unlock in: Cosmos Chance Draw (Limited Event).
Battery - Torque
Unlock in: Battery - Torque Black Friday Crate.
Call of Duty Characters: Ghost
Real name: Simon "Ghost" Riley.
Character appear in Call of Duty: Modern Warfare 2, Online, Heroes, Ghosts, Mobile.
Unlock Ghost in Battle Pass season 1 permium battle pass tier 1.
All Call of Duty Mobile Ghost skins:
Ghost - Plasma
Unlock in: Ghost Plasma Soldier Crate.
Ghost - Stealth
Unlock in: Ghost Soldier Crate.
Call of Duty Characters: Merc
Full name: Mercenaries.
Character appear in Call of Duty: Black Ops II, Strike Team.
All Call of Duty Mobile Merc skins:
Merc 1
Unlock in: Prophet Soldier Crate.
Merc 2
Unlock: Reach Master 2 Rank in Battle Royale season 2.
Merc 5
Unlock in: Ghost Soldier Crate.
Call of Duty Characters: Prophet
Real name: David "Prophet" Wikes.
Character appear in Call of Duty: Black Ops III, Black Ops 4.
All Call of Duty Mobile Prophet skins:
Prophet - Sight
Unlock in: Prophet Soldier Crate.
Prophet - High Voltage
Unlock in: Prophet Soldier Crate.
Call of Duty Characters: Ruin
Real name: Donnie "Ruin" Walsh.
Character appear in Call of Duty: Black Ops III, Black Ops 4.
All Call of Duty Mobile Ruin skins:
Ruin - Jade
Unlock in: Battle Pass Seaon 2 premium battle pass tier 25.
Ruin - Jungle
Unlock in: Limited Time Offer.
Call of Duty Characters: Special Ops
The speical ops characters collection are:
Special Ops 1
Unlock: Free.
Special Ops 3
Unlock: Free.
Special Ops 3 - Winter
Unlock: Limit time offer.
Special Ops 5
Unlock: New player login Day 5.
Conclusion
This is just a collection of the characters that has a lot of skins to help to choose the skins easier. Character like Gaz, Price, Terrance Brooks and a lot more also available in Call of Duty Mobile.