TL/DR: Don't abuse your friend's toys, play the Lost Soul faithfully to the character, accept that the prior player should be deferred to where possible, and there shouldn't be any problems. Where RPing another PC's character's actions are a "Ask Permission" situation, RPing a Lost Soul's is more of a "Beg Forgiveness".
If people are looking for a trump card to let them be obstinate and just "do what they want", then their attitudes need to be checked. SWC's mechanics support adversarial positions between players, while RP itself is about collaboration first and foremost, and that includes RPing other people's former characters. Keep those two mindsets in their proper places, and you'll likely be fine.
The Admins are hands-off with respect to RPing a Lost Soul, and would only ever review on a case-by-case basis. Syn's answer in another similar thread
is useful reading, but the moral of the story is there is no perfect, straightforward answer beyond the very general 'Do Unto Others - all the rest is commentary', and if you act unreasonably, then the former player has a legitimate complaint against you and punishment could theoretically happen.
I parse it all (several threads linked, my own memory, etc.) into this set of guidelines for Lost Soul RP:
1) Gaining possession/ownership of a Lost Soul does not grant any "absolute" rights to RP that character in a way its prior owner objects to on reasonable grounds
. All the same rules of respect among fellow players should apply: LS-Owners should figure out what they want to RP, run it by / consult with ex-PCs, follow their directions, and defer to them if there is a disagreement
in the collaboration. Ex-PCs should accept they no longer have the right to just blanket refuse the RP at all and must either collaborate promptly and reasonably, or take a back seat and rely on their remedy of complaining after-the-fact. Ex-PCs also see #3.
2) In many cases, the previous player of a Lost Soul is not available, is unresponsive, otherwise refuses to collaborate, or unreasonably refuses to condone requested RP scripts. Former two happen a lot, and shame on them in the latter cases. Owners or Commanders or Supervisors who have physical access can feel free to 'declare-as-canon rp' actions and speech of the Lost Soul, with the proviso that best efforts should always be taken to play the character in a manner that is consistent with the prior player's RP.
i) Also, it's usually a good idea to only RP a Lost Soul within the confines of its existence in Darkness where possible. If it's a prisoner, then RP it as a prisoner. If it's fighting alongside you because *mechanics*, and that's consistent with RP, okay then. If its Darkness existence is wholly inconsistent with the RP, either find a reasonable bridge to get the NPC from A to B or just Do Not RP them out of appropriate contexts. Put yourself in the former players' shoes and see the Lost Soul as a full character - not as a prop, and you should be fine.
3) If you are a prior player of a current Lost Soul, you should collaborate with the current owner on directing RPs, and not be unreasonable in your objections (e.g. you may not 'blanket' deny RPing. If you have an objection to the way your Lost Soul has been RPed, you should:
a) attempt to resolve things with the player who RP'd your former character in an amicable and collaborative fashion; and
b) if necessary, bring the complaint forward to the appropriate staff moderator (e.g. Scenario Mod, Holocron Admin, and all else failed ASIM)
@Belloq: I see no reason to make a distinction between owner and assigns, as with any "freedom" comes the obligation to be reasonable in any event. I think that physical access and Darkness Existence of the Lost Soul is far more important a distinction to make.