These can all be done outside of the Campaign (unless written otherwise), in Sandbox, and using mods, so go nuts and use whatever you want to get them.

• Billy Bones Jones

Unlocked by digging up the skeleton in Lee Chemicals, located in the toxic pool near the escape vehicle. It can be broken, so be sure to only use the sledge to break the dirt under the water.
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• Gina Campbell

Unlocked by destroying both ooP stores.
The first is located in Evertides Mall.
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And the second is located in the final area of the game, Cullington. Hard to miss.
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• Sam Sandstream

Spray paint the tanning machine in the Sunny Max store, located in Evertides Mall.
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• Chris Seabranch

Pretty simple, just blow up the hot dog stand in the Marina.
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• Magnus Enstöring

Also very simple, you need to destroy the light in the lighthouse at Hollowrock Point.
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• Ingrid Funke

Destroy 80% of the food court and escape with under a second left in the mission Covert chaos in Evertides mall.
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• Monty Quilez

Deactivate 100 robots by dumping them in the water. You can use the spawn menu to spawn them over the water and let them drown. Might take some time. It also doesn't need to be done in one map, just overall.
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• Parisa

Beat the Campaign's final mission. That's all.
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There are two other characters to unlock, but these are if you have the DLCs, and both of them are to just finish or start their respective campaigns, which you should have done if you own them in the first place.

Uhh yeah bye