The Technomancer features the ability to romance a couple of your companions and with that I have a full guide on how to gain the love of Neisha. Below are the steps and key points where I gained extra relationship bonuses, these will be highlighted as they're the most important to gaining a "lover" relationship with her.
When you've completed the disappointing fade to black romance scene it will unlock the achievement "the spy who loved me" for 30 gamerscore or platform equivalent. Quick facts: Neisha is introduced through story so it's forced you meet her though relationship wise you do need to take some initiative, there will be spoilers as well below though will try to keep brief to not ruin the story. I've also included a map of where you come across Neisha in Ophir.
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-After playing for a bit you'll come across a section where you need to save a group of officials, this is where you first meet Neisha.
-After completion of the escort mission go talk to her by heading to the "The Source" area which is located in the second level of the core area which if you're near the start of the game will find it apparent. There's also a map I've included in the above image to find her if you're having troubles.
-Neisha asks you to help out her friend Abbie in the local market and this is the first side quest she gives you.
-While there talking compliment about her career, ask about her life and she'll describe it.
-Go see Abbie in the market, it isn't far and it's clearly indicated on your map. Head to the close base, talk to the soldier that's been causing issues. Search the close bags again marked on the map and ask the Captain about Abbie.
-Then after completion of this head over to the market area and you'll hear the soldiers talking about teaching her a lesson. Intervene and teach them a lesson and you have a couple hours in-game to do so. Abbie will thank you for doing so, then head back to Neisha and report the good news.
-In the desert later on ask her about what she needed to mention earlier and this starts a quest.
-Ask her personal questions: "about music", about "herself/being alone" and "about being a spy". She's a bit odd as it's harder to get bonus relation points so it's really about just doing the quest for this gal.
-Do her side quest back in the slums, intervene on her actions and nothing negative happens.
-Later on compliment her, use tenacious for +2 relationship
-When in Noctis later on agree to go talk with Gloria (her friend) and deal with that.
-This will stem another quest, work on that and eventually you'll get a I'd like to chat with you alone or along the lines of next time we're at your place.
-Find a bed, kiss her and then choose to stay in the relationship. The achievement will then unlock and you will be "in love" with the character.