Squadmates fight for the Leader. Squadmates can either be captured after a battle, or recruited from the Mercenary Recruiter (Mr. Moustache).
There is a limit to the number of Squadmates you can have, if you recruit any Squadmates above this limit, they are placed in your Gifts>Temporary tab for 24 hours. Be sure to perform the Fatal Fusion or Discharge Squadmates you don't need to keep your ranks small enough to recruit new ones.
Squadmates by rarity
Squadmates by Strength
The stat based Tier Lists cover epic squadmates, while all of the lethal squadmates are placed together on one general tier list. Check back here often for Updates!