Billy and Jean is a two-headed ram that you can find behind the Hoon Homestead, east of the Ranger HQ on the Colorado map. Use Animal Whisperer to get the ram to follow you and make sure it stays alive until the end of the game. If the animal dies, make sure you reload your save.
EDIT: Thanks xX CHUCKLZ Xx for pointing out that if you choose to save the power armor convoy rather than the Hoon Homestead when given the option, you will miss out on this achievement.
EDIT 2: I finally unlocked this achievement and I can confirm that you do NOT have to have him in your party for the entire game. I actually never went and charmed Billy and Jean until after I captured Victory, which is 2/3 of the way through the game. The achievement unlocks at the very end of the game when your achievements for completing the game unlock. I would just make sure you have him in your party BEFORE you capture/kill Liberty, as the game is on rails after that point and you may not get the chance to grab him .
You should also make sure that you charm him with one of your created Rangers because you may lose him right before the final battle if your companion decides not to join you.