You don't have to be alone fighting your way out of the underworld. Hades actually has companions, which are items that can summon a friend into battle for a short span of time. Such friends can be useful for boss battles, and the companions can be leveled up with Ambrosia to increase the number of summoners per underworld escape attempt. To fully upgrade a companion costs 10 Ambrosia in total to increase its maximum summons to five.
Companions in Hades are equippable in the courtyard and unlike Keepsakes, they cannot be changed mid-run. They also can only be used once per chamber. There are six companions in all. None of them can be used against Hades himself unless he gives permission.
6 Antos
.jpg?q=50&fit=crop&w=1500&dpr=1.5)
How To Obtain
Antos is obtained by reaching a high friendship with Achilles. To get there, six nectar are needed to get to his favor quest. Once that is done, his next heart is locked until his favor is completed. That favor is to reunite him and Patroclus. This will take some chatting with Achilles and Patroclus until Nyx helps Zagreus find the pact Achilles made with Hades.
Zagreus will need to alter the pact to let him visit Patroclus in Elysium. This will cost five diamonds. Once that is done though, he will visit Patroclus and his final heart will be unlocked. Give him Ambrosia and he will give Zagreus Antos.
How Antos Works
Antos summons both Achilles and Patroclus to help Zagreus. They deal 1,500 damage to two enemies (one after the other). So the companion is best used when there are at least two enemies in the chamber.
5 Battie
.jpg?q=50&fit=crop&w=1500&dpr=1.5)
How To Obtain
Battie is a product of foraging a strong bond with Megaera. After gifting her six Ambrosia, her affinity will be locked until her favor is completed. That favor is less of a mission and more of a random event. Megaera will eventually appear in Zagreus' room to tell him that she only fights him because she has to (this only occurs after the time she collects her things from the room).
That event will unlock her hearts. Give her an Ambrosia after the favor and she will give Zagreus Battie.
How Battie Works
This companion summons Megaera to fight. She deals 2,500 damage in a line in front of Zagreus in the form of AoE circles. The companion cannot be used during any Fury boss fight.
4 Fidi
.jpg?q=50&fit=crop&w=1500&dpr=1.5)
How To Obtain
Fidi is given from having a high affinity with Dusa. Six bottles of Nector are to be given to Dusa before her favor is required to be completed. Dusa's favor is to complete 12 lounge-related renovations with the House Contractor. These have to include cleaning up Cerberus' mess and adding a rug.
Once her favor is complete, give her a bottle of Ambrosia and she will gift Zagreus the Fidi companion in return.
How Fidi Works
Fidi summons Dusa to Zagreus' side to fight. While she is in the fight, she will fire 70 points of damage at enemies and the attack will freeze them for 30 seconds.
3 Mort
.jpg?q=50&fit=crop&w=1500&dpr=1.5)
How To Obtain
Obtaining Mort requires getting close with Thanatos. Give Thanatos six Nectar to get his favor quest. The favor is actually not one that is mentioned but is completed once Zagreus completes a narrative that plays out between them during a number of their encounters with each other. The favor is finished when Zagreus proves his strength to Thanatos.
Thanatos' then can be given Ambrosia and he will give Zagreus Mort.
How Mort Works
Mort summons Thanatos to battle. Thanatos will deal 3,500 damage in a circular AoE in front of Zagreus.
2 Rib
.jpg?q=50&fit=crop&w=1500&dpr=1.5)
How To Obtain
Rib is a gift from Skelly. To obtain Rib, Zagreus must give Skelly six Nectar and complete his favor quest. He will directly request his favor, but won't give it unless Zagreus has unlocked four non-Zagreus aspects of the various weapons. Once that is done, Skelly will ask Zagreus to kill him with the level five Zagreus Aspected Stygian Blade (which will require five Titan Blood to level up).
Once that favor is done, give Skelly an Ambrosia and he will gift Rib to Zagreus.
How Rib Works
Rib summons Skelly who will distract enemies and draw their attacks away from Zagreus. He remains up until he is killed. He can take friendly fire from Zagreus. The companion will not work in fights against Charon.
1 Shady
How To Obtain
Shady is given by Sisyphus. To obtain it, give Sisyphus six Nectar and complete his favor. To do this, Zagreus will need to void Sisyhus' sentence. This is one of the most difficult favors in the game to complete. Zagreus will need access to the Administrative Chamber (can be bought from House Contractor), five hearts with Megaera and a conversation with her about Sisyphus, five hearts with Nyx, and a conversation with her about the Admin Chamber, and then a random follow-up discussion with Nyx about Sisyphus. Once all those requirements are met, Zagreus can find the document in the Admin Chamber.
However, the favor is not complete yet even then. Four diamonds are needed to pay the House Contractor for Sisyphus' freedom. Then find Sisyphus and tell him the good news. Give him an Ambrosia, and he will give Zagreus Shady.
How Shady Works
Using Shady will have Sisyphus drop Bouldy to the ground in front of Zagreus. This deals 1,000 damage in an area and drops an arrangement of items such as health, gold, and darkness. The companion is extra good when Zagreus has the Ocean's Bounty boon.