The exact definition and function of Nexus Beings is some what confused. At times they have been described as Beings who "exist in every conceivable timeline" (which is unlikely) and other times they've simply being referred to as a being who can in some way affect the destiny or future of their Universe in a hugely significant way. But at the same time it's worth noting it was said that the Nexus Beings in question are not necessarily the most prominent beings in their Universe (although it has been the case). It's also not entirely clear how a person's standing as a Nexus Being in their reality translates to their counterparts, for example many of Kang's divergent selves were named as Nexus Beings but Spider-Man of Earth-772 is a Nexus Being whereas his Earth-616 counterpart is not.
Kang, Merlyn, Scarlet Witch and Sise-Neg were all confirmed as Nexus Beings by Uatu the Watcher. These four were Nexus Beings on account of their ability to in some way alter the Timestream (be it through time travel or a probability hex). A woman called Lore, from an unidentified reality, was another Nexus Being who travelled the Multiverse killing other Nexus Beings. Other confirmed Nexus beings include Odin of Earth-9260, Spider-Man of Earth-772, Ultravision of Earth-90110 and Wolverine of Earth-9250.
Uatu the Watcher also stated Leonard Tippit was a Nexus Being, although it has also been suggested that he simply had tapped into the Destiny Force. Similarly, Rick Jones was described as a Nexus Being for having also tapped into the Destiny Force, what the exact connection of the Destiny Force and a Nexus Being is has never been explained and it simply might be nothing more than a coincidence that the only two people to ever access that power were both supposedly Nexus Beings. Havok was referred to as a Nexus, specifically the Nexus of All Realities, but similarly was never fully explained.