Titan Tails (Archie's Sonic the Hedgehog) is the result of Tails merging with all of his other-dimensional counterparts across the multiverse.
Persona Vabel (Accel World vs. Sword Art Online) is Yui from 1000 years into the future, who was ultimately merged with her past self; due to the conflict of memories, the new Yui suffered a memory reset.
While confronting Sora in Scala ad Caelum, Master Xehanort (Kingdom Hearts III) merged with his other selves that composed the true Organization XII, becoming Armored Xehanort.
After traveling back in time to save Kiya from Jack the Ripper, Sir Daniel Fortesque (MediEvil) merged with his past self, gaining a protective suit of golden armor.