Getting a little ahead of Jenayah, could this be Huntik: Secrets & Seekers as per Anime with a boy hiring a creature from a stone, meets a man named Dante and starts a journey to collect crystals?
The series follows Lok Lambert as he uncovers the secret legacy of his missing father. His father, Eathon, was a member of the Huntik Foundation and a part of a secret world of magic users known as Seekers and creatures known as Titans. On his mission to uncover what happened to his father, Lok is joined by Dante Vale, Sophie Casterwill, Zhalia Moon, and a talking Titan called Cherit.
Season one focuses on the fight against the evil Organization, led by a powerful Seeker known as the Professor. Along the way, the team discovers the Amulet of Will and the powerful Legendary Titans. Though the Professor eventually acquires the three Legendary Titans of Mind, Body, and Spirit in a bid for immortality, he is defeated by the Huntik team.
In season two, an ancient and evil group known as the Blood Spiral reveals itself and seeks to summon evil and destructive beings known as the Nullifiers. This will have ramifications for everyone, with the Blood Spirals being linked to the destruction of Sophie's remaining family, the descendants of the powerful Lord Casterwill. A new hero, Den Fears, joins the Huntik team as Zhalia goes undercover within the Blood Spiral. A new set of Legendary Titans are uncovered, but their discovery only heralds the resurrection of the Blood Spiral's founder, the Betrayer.
As per Anime that has pendant/necklace that transform to monsters, he has a pendant to a green one that has wings that fuse with him, the main character sometimes does indeed transform via the amulet.
It's more about summoning the creatures than merging with them, but it does happen.