Nectar

Nectar is one of several Artifact currencies that can be rarely found within the Underworld. It is preserved after dying in the Underworld. It can be given to characters as a gift to improve your affinity with them. Most characters are able to be gifted Nectar, except for Alecto, Tisiphone, Theseus, and Asterius.

Characters receiving their first Nectar from Zagreus will return the favor by giving him a Keepsake. Furthermore, gifting a character 5-6 Nectars in total will improve that character's trust in Zagreus so much that they will offer a quest. Completing said quest will unlock the rest of their hearts, which can be filled with Ambrosia.

(Additionally, as much as Bouldy would like to give a Keepsake, as an inanimate object he is unable to do so, though he may be gifted infinitely.)

With the Vintage Nectar upgrade from the House Contractor, Nectar will also give a random boon +1 Level when collected from a room reward. (As if Zagreus had picked up and used a Pom Slice.)

Codex entry

'...The drink of the gods is one of the Underworld's most sought-after delicacies, and truly perhaps one of the only sources of genuine lightness here within this realm. How the substance came to be here is unclear to me, though even I have something of a taste for it by now, based on the few occasions when I've had a chance to try it for myself. Everyone I know in the Underworld, irrespective of their disposition, categorically enjoys the stuff. It is a prized possession, and anyone who would offer it up willingly does so genuinely as a friend.'

Nectar Uses

Nectar is used to:

Obtain Keepsakes and raise your affinity with characters (149 )
Obtain Heart of Stone boons from Bouldy infinitely
Buy the Woodland theme from the House Contractor (100 )
Trading with the Wretched Broker
Purchase ranks in the Resource Director (50 )

Obtaining Nectar

As a Chamber Reward
As a rare extra reward for clearing Infernal Troves with a 5% chance
From the Fated List of Minor Prophecies (3 )
In trades with the Wretched Broker
Poseidon's boon Sunken Treasure has a 10% chance of dropping a Nectar
Dionysus' boon Premium Vintage will give Zagreus one free Nectar when chosen, and any Nectars found after that in the run (including the free Nectar) will raise
Fishing in Elysium and turning in the fish to the Head Chef (1/2/3 )

Notes

Prior to patch 032, Nectar had been known as Ambrosia.