You acquire D-Horse at the start of Mission 1. As with the other buddies, using her on missions will increase her bond with you. Once you max her bond rating, the achievement will unlock.
You can check your bond level with your buddies by highlighting them on the Select Buddy option in the Mission Prep screen, or upon completing a Mission on the bottom left of the ranking screen.
As you increase the bond with D-Horse, you will unlock more commands and equipment to use with her. To use her commands, hold
and use the
to choose a command. Click the
to issue that command.
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