Source? I think its pretty obvious that he's a spell, but does that mean the spell left him without Personality? This is magic we're talking about, so unless its actually said, we can't really assume that. After all, think of all the creatures in LoZ, let alone all of fantasy, that were created through spells yet have their own Persona.
Also, this goes with the earlier point. A new Dark Link can easily be created through other means that would give him a Personality. We have multiple Links, multiple Zeldas, and even multiple Tingles, I don't see whats wrong with making Dark Link different from his past selfs.