Wasteland 3

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.

As GooseBoxLive said, make sure to tame Billy and Jean and have him in your party before entering the last door and confronting Liberty. Make sure to have the hard-ass skill at 10 in order to avoid fighting her and not risk the Ram being killed in combat.

After you have captured Liberty you have two options. Side with Angela or side with the Patriarch.

Siding with either of them results in you having to fight rangers at your base. The only difference is siding with Angela means you won't be fighting refugees in the downtown area. Siding with the Patriarch means you will be fighting the refugees on the way to final battle but have the option to avoid the final fight with a speech check. You will need a hard ass and kiss ass at 10 in order to arrest Angela when confronted. I chose to side with Angela and just run over the Patriarch and his guards in one turn. This exposes Billy and Jean to less battles. As soon as the end slides roll the achievement will pop if they survive.

When you first head to the world map you will be presented with two missions.

The Hoon Family residence or the Arapaho Caravan.

In order to get this achievement you must go to the Hoon Family residence.

Once you have cleared the enemies head behind the main house to find a two headed goat in a pen (Billy and Jean), tame the goat (Animal Whisperer rank 1) and then keep it alive as a companion until the end of the game.

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As an edit to this I have not yet tried leaving the goat at the Hoon residence until near the endgame and picking him up then so not sure if this works or if you must get him at the first encounter.