Solomorne Romance Guide
Gender lock: Flirt and kiss are open to any gender. The later go-to-bed step is reported to be female-only; the flirt-and-kiss path works regardless of how you play.
TL;DR: Solomorne, the Lex Imperialis DLC companion, opens to flirtation the moment you board the Heartless with him in Act 2. Take every flirt option, compliment him to crack his armor, and keep choosing the caring lines through the DLC even when he refuses at first.
The short version
entity:solomorne Anthar is an Adeptus Arbites officer, and romancing him is exactly as hard as that sounds. He is loyal to the Lex Imperialis and to his dog, and little else moves him at the start. The relationship is a long campaign of flirtation against a man who answers with stoic resistance and the occasional eye-roll before he finally lets you in. Persistence is the whole strategy, and there is no conviction requirement to begin.
Worth knowing up front: romancing Solomorne is the only way to unlock a specific quest that otherwise stays hidden. Even if you are unsure about committing, taking the flirt options keeps that content on the table.
Where it starts
entity:solomorne joins in Chapter 2. After you clear Janus or another main world, you receive a message from him and recruit him at Footfall. In the Lex Imperialis DLC, the romance window opens during the quest "Heartless Void" from the moment you board the Heartless alongside him. From there on, whenever you talk to Solomorne, a flirt option tends to appear, aboard the Heartless and later back on your own bridge.
Romance steps
Flirt at every opportunity. Whenever a conversation with entity:solomorne offers a flirt option, take it. These recur across the DLC's quests, on the Heartless and on your bridge, so the romance is built one repeated choice at a time rather than through a single decisive scene.
Compliment him. Calling him charming and otherwise praising him is how you start to crack his hardened exterior. He does not soften quickly, so lean into it.
Wink, then take his hand. Use the "Wink" option, then "Take his hand" to go to bed with him. The sources split here: one walkthrough reports the bed scene may be possible only as a female character, while other accounts hold that he is romanceable for both sexes. The confirmed path is that the flirt-and-kiss track is open to any gender, with the bed step reported as female-only. Pick the options either way, because the romance still progresses and the gated content still depends on it.
Keep picking the options even when he says no. Solomorne refuses at first. Advances meet stoic resistance and eye-rolls, and he ends up saying no early on. Choose the romance options anyway. The refusal is part of the arc, the choices come in handy later, and the romance-gated quest only appears if you have been pursuing him. He eventually relents.
Stay the course. Through the later conversations, keep selecting the "I care for you, Solomorne" style romance lines to keep the relationship alive once it has finally taken.
What to expect
Solomorne is written to be difficult on purpose. He can be moved by his dog and by the Lex Imperialis, and human affection of any kind is a hard sell. The payoff is twofold: you win over the one companion who seemed immune to it, and you open a quest that no other path will reveal. Treat the refusals as the cost of admission, not a dead end.
Exclusivity
The standard rule holds: you can keep more than one romance running until the quest The Lord Inquisitor and The Lord Captain, then you settle on a single love interest. If Solomorne is the one, keep feeding him the caring options through the back half of the DLC so the romance is firmly established by the time the choice point arrives.
For where Solomorne's Overseer kit lands as a party pick, check the companion tier list and the builds hub. For the other DLC and base-game flames in one place, see the romance guide.