A complete list of all allies known so far along with their stats can be found here
General
There are multiple ways of encountering survivors. Some are always there, some are time-based, some are random, and some will find you.
You are not forced to keep allies, but it's really hard to survive without them
There are no children allies.
There are dog allies. They take up an ally slot in travel mode. They are great at warning you about dangers you can't see and they are also effective against humans.
Allies will frequently bring areas to the player's attention, unlocking them on the map. Locations can be found without allies, so you're not cut off from content if you're missing an ally with that info.
Allies can panic in combat. Some allies are afraid of zombies , some are afraid of getting hurt, some panic at just leaving the shelter.
Each ally has perks specific to them, usually based on their skills/former jobs.
Allies gain skill points differently than the player. They receive 1 skill point for every day they are alive, and will sink that into a predetermined character path. There are ways of influencing their perks/focus, but not directly arrange their skill/stat points.
Allies may be in relationships, but any relationships you might have with someone are fleeting.
Sometimes confrontations between two allies can escalate to the point that the player needs to get involved or risk major consequences.
Allies that don’t like each other may lose confidence in you if you're not able to convince them it's a good idea to have the other ally around.
If enough people want you out and there's someone willing to lead them, then you may be overthrown as leader. It's a game over situation, so keep an eye on your allies.
Allies with own and complete page
Aggro Moss, Aimee Sysko, Anita Cass, Bud Larsen, Clifford Ghaffey, Davis Cray, Doug Schultz, Effrem Montague, Elaine Martin, Fiona Parks, Grant White, Jenifer Strong, Jodie Hopkins, Joel Oswalt, Lane Vibesit, Lightning, Lloyd McCroskey, Mark Corbeau, Max Getz, Melina Guzman, Mia Seza, Michiru Yata, Nathan Harris, Oscar Guzman, Paul Rainier, Priscilla Butler, Regina, Renee Cass, Ryan Michaels, Sandy Doubleday, Sara Seza, Todd Kantell, Vic Castillo
Allies currently missing information
Carlton, Craig Sullivan, Darlene Lumpkin, Gillian DeVries, Harold Kyun, Isobel Dee, Karen Landey, Nash, Parisa Sharazi, Troy, Ulysses Ford, Walter Addison
Allies with information, but incomplete or no page
Brian
Brian Doublebear himself is not a character in the final game; however, his portrait and model are borrowed from an ally who has yet to be introduced to the game. Brian is a temp character used by the player in Dead State's pre-Update #3 Early Access.
Sir Charleston
Found in random encounter it a nutter with armor and a sword who thinks he is a knight