Hakkari and Suki's souls are the same. King Avados created an exact replica of his daughters soul and created both Hakkari and Suki using copies of that soul.
Connection between Hakkari and Suki
When Hakkari and Suki meet, they instantly feel a bond with one another and slowly become great friends. Later on, Hakkari finds out that Suki was one of the apostles of King Avados. Suki then forces Hakkari to fight her and after an intense battle and hakkari awakening hidden powers of death, she succeeds in beating Suki, which was all part of Suki's plan.
Before Suki dies she passes her bladed enchanted wings, the "Everblade Wings", to Hakkari. Legendary Enchanted equipments usually need to be crafted and fined tuned to work specifically with someones soul but since Hakkari has the same soul she was able to wield the wings. Suki tells Hakkari that they will always be best friends and told her about how much she admired and cared for King Avados like a father and she only wanted to help him for all that he has done. Hakkari doesn't quite understand what this means but she knows there must be more to King Avados then just a tyrant who seeks to quench his boredom.
Later on when Hakkari fights Suki's brother, the brother fights her full of hate and revenge. The brothers wings are too powerful but then Suki reveals herself to Hakkari, who is puzzled. Suki kept her soul within Hakkaris and tells her how their souls are connected and made from king avados as copies of his daughter. Suki then convinces Hakkari to let her merge their souls together and that she would be stronger in doing so. Once they merge Hakkari's power is doubled and is able to produce a lot more bladed feathers from her wings. Hakkari manages to defeat the brother and convince him that Suki is still alive. He then joins her to help put a stop to the king.
Near the end, during the battle against the king, the brother is severely injured and convinces Hakkari to combine his wings with hers to transform the wings into their true form. A 4 winged enchanted artifact, one set has the power to telekinetically control all the bladed feathers and teleport/switch places with any of the feathers. And the other set, which has the power to shoot beams of light and move very fast. Together both sets share the same abilities between each other and produce a way higher output.