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 usually 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 and then gifting the character an Ambrosia will unlock a Companion as a reward.

(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.)

Besides this, Nectar can also be used to purchase the Theme, Woodland from the House Contractor, or traded to the Resource Director (a total of 50) at various ranks to rank up.

Obtaining Nectar
Nectar can be received as a room clear reward, from Charon's shop for 200, as a rare extra reward for clearing Infernal Troves with a chance, from the Fated List of Minor Prophecies, or in trades with the Wretched Broker.

Poseidon's boon Sunken Treasure also has a 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.

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

Fishing in Elysium is also a way of obtaining Nectar.

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