I found Odessa to be the first available, she also has a bit of a story to her. You first find her character locked up on "Ithaka", second island you visit after the starter area. She's in the middle of small fort area "Odysseu's Palace". Help her out, always pick the heart icon in dialogue choices. I added "to follow her dreams" in the last dialogue choice before she leaves.
You see her again later in the next area, the first big place you sail to. After some quests you'll find her by some pig pens in "The Valley of King Lelex". You'll need to do small quests; gather close herbs, take out rebels and then go grab documents from a close house. After that, you're good to go. Pick the romance choices like mentioned prior and before the final dialogue choices the achievement should unlock. She's also a great addition to your boat crew. Hope the maps help, courtesy of Gamerheadquarters
. I'll also mention that other moments that are quicker can be found, this was just the initial one that I came across.