The first thing I can think of is creating a Barbarian or Bloodrager and reflavouring rage or bloodrage as another personality taking control, but what other options are available?
(Inspired by Fei Fong Wong from Xenogears)
Vigilante (with differing aligments, even!)
The Medium can be reflavoured too.
There are a couple of Jekyll and Hyde or Bruce Banner/Incredible Hulk archetypes. Vigilante has one called Brute that makes you have to make Will saves in order to not attack your allies.
The Abomination Psychic Discipline for Psychics might work.
If you're going to go with the reflavouring approach, an Alchemist's Mutagen or Cognatogen could work.
Medium and Spiritualist both have potential, too. The Fractured Mind Spiritualist archetype even makes your Phantom an extension of your own spirit rather than the spirit of someone else.
Kineticist with just the first Elemental Whispers talent can basically talk to a voice in his head
Split/multiple personalities is an insanity in the GMG.
Master Chymist.
THANK YOU I knew that one but I couldn't remember where it was, what it was called, or what it was for!
I support Bloodrager as a good choice if you want to have something similar to Id. Specifically, because he was stronger and had access to extra power.