Help me with implanted cohort cheese

[Analytica]

I am toying with an idea of a psionic character that has some form of psionic entity bound into it. Aside from the jinchuriki flavour, I see two nice crunch applications:

- Thieving mindlinking the cohort to access more powers known
- Metaconcerting the cohort to super power up in dire situations
- The cohort occasionally acting independently from within the character

The question, then: how can I do this without homebrew? Ideally, I would want a creature (that can take psionic class levels, so as little LA and RHD as possible) that can remain indefinitely within the main character. Bonus if it actually cannot live outside it. Incorporeal, possessing creature would be ideal, but I also considered things like an elan fetus within its elan mother (maintained indefinitely as they do not age), or a similar case with a vestigial twin. Are there other options for this that you can think of?

[Flickerdart]

Leadership for a Fiend of Possession is the easiest way.

[Analytica]

Yes, that's good. Any way to make the cohort an outsider without too many lost manifest levels, except for neraph?

EDIT: Actually, FoP says the creature "cannot be targetted by spells or attacks separately from the victim". Might this mean it is not a valid target for Thieving Mindlink? I assume you have line of sight to something which is a part of your own body. Of course, if I am being that RAW OCD about it, I guess that line does not actually mention powers at all, and RAI it is part of a discussion on whether the FoP can be damaged, so maybe that isn't really a problem.