You have to complete the questline of Eugenie for this achievement.
Talk To Eugenie (in fact with every NPC) at every opportunity. (You can check at Fast Travel point to see if any NPC has any dialogue or side quest).
It's Game long side quest. (I played Lie playthrough first and then Truth Playthrough in NG+ where I got this achievement. And for this achievement you need to tell the truth )
1. Get Gloves from Eugenie for Alidoro after chapter 7 before going to Barren Swamp area for Chapter 8. You will notice Alidoro is not at the hotel.
Timestamp: 04:28:00 (Part 3/ NG+ video)
2. Give Gloves to Alidoro at Barren Swamp area. he is standing at unmissable path in Barren Swamp.
Timestamp: 04:46:55 (Part 3/ NG+ video)
(Hugo will also give some dialogue and a pamphlet about Alidoro, about which you can actually ask from Alidoro directly.)
3. Talk to Eugenie at Hotel, tell her truth.
Timestamp: 05:03:00 (Part 3/ NG+ video)
4. After defeating Black rabbit Brotherhood the second time in Chapter 10: Relic Of Trismegistus, you will meet with Alidoro again. Exhaust all dialogues with him and in the end kill him with one hit. You will get Alidoro's Cryptic Vessel.
Timestamp: 06:49:00 (Part 3/ NG+ video)
5. Go to Hotel and First thing is to Decrypt Alidoro's Cryptic Vessel with Venigni and only after you talk to Eugenie again.
Timestamp: 06:58:05 (Part 3/ NG+ video)
6. Tell her the truth 'He was Eugenie’s older brother'. Then simply 'use the stargazer', you don't need to fast travel anywhere.
Timestamp: 06:56:25 (Part 3/ NG+ video)
7. Talk to Eugenie again. Exhaust all dialogues with her. You will get a record 'Proposal, Flower, Wolf Part 1' and this achievement.
Timestamp: 06:58:25 (Part 3/ NG+ video)
(Note: If you are in your lie playthrough, you will not get the achievement but another record 'Far East Princess'. And you need both records for Golden Melody achievement.)
100% Complete Walkthrough for all achievement/collectibles and Full Game: