Venus in the 12th house synastry means their Venus falls in your 12th house: where their affection lands in your life. What it feels like from both sides, the gift, the shadow, and how to find your overlays.
Their Venus in your 12th house is the quietest Venus overlay: their affection lands in the house of sleep, secrets, sacrifice and everything you do not say out loud. You may not be able to explain why you feel so tender towards them. They tend to love you privately, sometimes without telling you, and the bond often feels older than the relationship.
Their Venus in your 12th house
Something in you softens around them that you cannot point to. Their Venus touches the part of your chart that holds dreams, old grief and the things you have not said, so they may know how you are feeling before you do, and you may find yourself crying in front of them early and not minding. You feel forgiven by them for things they do not know about. They can also be hard to read. On their side it is stranger still. They are drawn to you without a clear reason, often feel protective, and may keep their feelings hidden for a long time, from you or from themselves. Secret relationships, love that begins while one of you is unavailable, and quiet devotion that outlasts the relationship itself are all classic 12th house stories.
The gift
Being loved without having to perform. They accept the parts of you that you thought disqualified you, and the acceptance tends to work on you slowly, like sleep. Compassion runs both ways: you are gentler with them than you are with most people, and they with you. Shared silence is comfortable. They get a partner who brings out their capacity for devotion, and a tenderness they may not have known they had. Creative and spiritual work often flourishes between you.
The shadow
What is not said tends to rot. Both of you may assume the other knows, and build on feelings that were not checked. Sacrifice is the 12th house trap: they may give up too much for you and resent it later, or you may keep them half-hidden from your life, an affair in spirit if not in fact. Idealisation is common, and the real person under the tenderness can come as a shock. Some of these relationships exist mostly in one person's head.
Working with it
Say the thing. Every so often, tell them one feeling you had assumed they knew, and ask them for one in return. Bring the relationship into daylight: introduce them, put a photo up, let it be seen. Notice sacrifices as they happen, yours and theirs, and refuse the ones that are not asked for. Pay attention to your dreams while you are together, since they often carry what the two of you are not saying. Rest near each other.