If I'm a rogue/fighter multiclass, and I attack with my echo knight (which is out in the open) while I am hidden, does the attack have advantage, and does it reveal my position?
You still have advantage
When you have hidden from a creature, that creature can't see you. Even though they can see your echo, the echo isn't you, it's just a magical, translucent grey image. Since you cannot be seen, you are an Unseen Attacker:
When a creature can't see you, you have advantage on attack rolls against it. If you are hidden--both unseen and unheard--when you make an attack, you give away your location when the attack hits or misses.
Narratively, this advantage simulates the difficulty in predicting the attack. This doesn't seem too strange, but it is unusual that your position will be given away. Normally if you make an attack from hiding, the enemy will be able to figure out where the attack came from. In this case there doesn't seem to be any way to know, so it doesn't feel right that your location is revealed.
Echo Knight requires DM rulings
As strange as it may be, those are the rules as written. The Echo Knight is a class with many problems, so you can expect your DM will have to make rulings so that everything makes sense.