Guts' New Party (unofficial name) is a party consisting of allies Guts came across after the original Band of the Hawk had perished. Only taking with him Casca from the original Band of the Hawk, his party slowly, but surely increases in size as the story progresses and the journey continues.
As Guts had originally planned to fight the God Hand and Apostles alone, he does not think of forming a party until Isidro, Farnese, and Serpico join, reminding him of his late comrades in the Band of the Hawk.
Puck helps Guts heal with his Elf Dust when he's wounded, and also serves as a comic relief from time to time. Puck initially assists Guts for (unintentionally) saving him from the Snake Baron's men. Later, after Guts kills Rosine, Puck tells Jill that he does not know why he follows Guts, having stayed with him through the slaughter of three Apostles. Guts finally allows Puck to travel with him when the latter fends off daytime ghosts from his heavily wounded body.
After the horrific events of the Eclipse, Guts left Casca (now suffering from dementia) behind at Godo's hut with Rickert and Erica. He would then take off to seek revenge against Griffith, the God Hand and their Apostles in what would lead into the Retribution/Conviction arc. Returning two years afterwards, he found that Casca was missing. Back at Godo's hut, Guts realized that going off fighting by himself was not the right decision, and that he would never leave Casca behind again. He then went on with Puck to the Tower of Conviction and rescued her.
On his way to the Tower, Guts would meet Isidro, who tagged along because he was amazed by Guts' swordsmanship and fighting abilities. Isidro's reason for tagging along was not to become Guts' apprentice, though, but to steal Guts' techniques. While mainly paired with Puck for comic relief purposes, Isidro has proven to be a vital asset to the party, saving Casca and the others at multiple occasions.
After the events at the Tower of Rebirth, Farnese, whose faith had been shaken before from seeing the evil spirits that haunt Guts, chose to abandon her position as leader of the decimated Holy Iron Chain Knights and instead follow Guts on his adventures. When the party reached the town of Vritanis, Farnese would reunite with her family. Guts then asked her if this was the last stop of her journey, to which she replied that there was no place for her among the nobles, and she only wanted to visit. Farnese's position in the group is more or less the caretaker of Casca, as she seems to be the one Casca is most attached to. Farnese has also taken up learning witchcraft from Schierke, so far having succeeded in forming the Formation of the Four Cardinal Points during the group's battle with the Sea God.
Serpico, Farnese's servant, was reluctant to follow Guts after the events at the Tower of Rebirth, but had little say in the matter. After Farnese made her choice in Vritanis, he too followed Guts as members of his new party. Serpico's main concern is the well-being of Farnese, and it seems as though it doesn't matter to him which side he's on or who he's fighting for so long as she's all right. Serpico demonstrates remarkable ability with the sword and has a penchant for strategy, choosing settings for battle that would place him in a strategic advantage over his opponent
Schierke is a witch who joined Guts' party after the death of her mentor, Flora. Only seeing the humans as savages and beasts at first, she grows to enjoy her human company and travelling lifestyle. Schierke was a powerful witch who can use her witchcraft to take out multiple opponents at a time. Before Flora died, she also handed the party several magical items for use against the spiritual monsters they fight along the journey. She can utilize a magical seal to prevent the brand from attracting evil spirits, and also serves as an anchor to call Guts back to his sanity in his use of the Berserker Armour.
Evarella is an elf who tagged along with Schierke. She is not from Skellig, like Puck, but shares his playful attitude. Just like Puck, she can emanate elf dust, providing heals for the party whenever necessary. Her individual function is to provide support and guidance for Schierke's magic, having been her supervisor prior to Schierke's involvement with Guts' party.