You will need to rescue Kelvin at the start of the game, and keep him alive throughout your playthrough. You will also need to earn the trust of Virginia, and make sure she survives the game.

Read More: How to recruit Virginia

Once you have got both characters recruited, with Virginia aiding you through her cat like gift culture, and Kelvin, you know, being Kelvin, you can then complete the game. Visit the final lab on the far east side of the map, then complete the lava tunnel to reach the cube. Reaching he cube while both allies are alive will bring them into the cube with you during the cutscene. If both character make it into the cube, then they can be rescued alongside you at the end of the game, granting you this achievement.