Bethesda Game Studios has made a concerted effort to improve its core relationships in Starfield, and that is very much apparent. In all, Starfield has 4 romanceable/super friend options (Sarah, Sam, Barrett and Andreja), each of which actually has its own Mass Effect-like loyalty mission at the end (although nowhere near that kind of quality).

Basically, to get a companion to the maximum relationship level, you need to take them everywhere with you, and make dialogue choices while in the game that they would approve of. In other words, when you’re with Sarah, don’t be a dick, don’t be evil, don’t steal, don’t kill innocents, and when you get a chance to speak, talk about how you love Constellation and exploring the galaxy. Just generally be as nice as possible.

Then, at various points throughout the game, the companion in question, whoever that may be, will ask to speak to you, and they’ll reveal more about themselves and open up more. This process then goes on and on: you do things that they like, and they’ll eventually ask to speak to you as you climb the various invisible tiers on the relationship tree - it'll even create an entry in your Activities list too (which will help with Fixer (30G)).

In terms of tips for how to max out a relationship, try these:

• If you want a romantic relationship, when you get the chance choose “Flirt” as the conversation option.
• Try to stick to one follower companion until you've unlocked this achievement. Once you’ve done one, then you can share the wealth and try the others.
• Different companions give you different gifts every so often - Sarah gives you organic resources, Sam gives you inorganic resources, and Andreja straight up gives you credits.
• Responses to things you do in the world range from this character “Dislikes that” and”Hates that” to “Likes that” and “Loves that.”
• If you’re going to do something dodgy, leave your companion on the ship, or ask them to wait around the corner while you go and do something shady.

Basically though, get them to the top level and finish their loyalty mission, and see the relationship through to the maximum and you’ll unlock this achievement.