Mariv's Garden (マーリブの庭) is an imaginary place created by Mariv, which is located within her subconscious and where Mariv's human form also resides. Auz also mentions the garden as "a virtual space created from many gifts". It's the only area where you can fuse, sell, and store/retrieve your Genes in the Story Mode. You can access the garden after you arrived at New Paleo, unlocking a portal near the bed of the player's house, and can also be accessed by shortcut.
After the Protagonist arrived in New Paleo, Mariv links the players subconscious in the garden while asleep, and told him that she can help by altering genes to enhance their abilities, as well as telling him that his mind is already connected to her subconscious, unlocking the area near the player's bed.
While the story progresses, the player will meet Elroux here for the first time and afterwards, all of the party members, Leary, Daisuke, and Alfred can access. Johannes can also access the garden to visit Mariv.
In some certain quest, Auz will be present in the Garden, confronting your team with a challenge to test if you are prepared to face the Child and its parent, The Entity. Auz also changed the scenery battle them above the clouds, much for the team's surprise.
After Mariv's death from avoiding the Child to hatch, you can still access the Garden but Mariv will be replaced by a hologram avatar, meaning you can still fuse, but mariv's hologram is silent.