Back 4 blood is a left 4 dead style zombie game that is a 4 player co-op. You do not have to have 4 players to play. You can play it co-op with friends or you can play alone. But if you want to you can have random people join if you do not want to play along. It is cross play and allows people on console to play with pc players.
Karlee, who is another personal favorite because she is the style that I like, gives the team an extra inventory slot and gives them 25% movement speed.
Walker, is great to improve stats and strengths.
Mom, revives teammates instantly one time during any level. If anyone is playing as her in her inventory, you get get extra support and she can basically heal you and bandage you up.
Doc, is a healer and does extra damages and she can heal you for 25% of health once per level. She can also boosts you through out game to help you finish the level.
Evangelo, can break out of grabs, and his main weapon is a melee machete.
Each character, you can make a load out and get better perks with whatever cards you are choosing. This game is a good game and I personally enjoyed it. I even went through beta and it was really worth it.