Lights already knows Sayu is Kira.
After Chapter 8 shows Light actively lying to Kazue when brought up evidence of Sayu being Kira, a few commentators have come to theorize that light knows Sayu is Kira and is protecting her either because he believes Kira is actually right all along or simply because Sayu is family and will protect her no matter what.
- Played with: in chapter 9 while he was shocked to find out Sayu was Kira, Sayu was quick to point out the various holes in his behavior as of late, such as feeding her relevant information to her easily, his numerous reasons that he should of suspected her and how he actively lied Kazue with a better cover story to invalidate her claims more. Convincing light he always knew she was Kira and Light seemingly concluding that that he was just lying to himself because he was in complete denial.
Soichiro will likely think Misa Amane is the main Kira mastermind when Her relationship with sayu comes to light.
With the considerable Age difference and monetary power dynamic of the two. It’s likely through his biased point of view of the “innocent” sayu that will justify this view and think Misa is manipulating sayu to take the fall or groom her to be an accomplice. It may be the only cover story L could use to convince him that she is involved in Kira
- Jossed: While he is very apprehensive about his daughter being the main Kira suspect, he doesn’t need to be convinced or sugarcoated with a false story in order to believe it due to the evidence L provides him, as well as Sayu hiding the fact that she’s been in a lesbian relationship from them, places doubt in his mind that he ever really knew his Daughter at all.
The threat of Andrew McLachlan will force Sayu to compromise more of her morals as Kira.
While we don't get much of McLachlan beyond an Establishing Character Moment, from what we gather so far he seems to be a Machiavellian schemer ala David Xanatos who is going after Kira after taking the death of Jinbei Tamura as a sign that Kira poses a threat to businessmen like them should the forumgoers see them as corrupt. With both him and L gunning for her, Sayu would have to change tactics in order to stay ahead of both of them, likely resorting to more morally grey methods and even changing more of her rules as a result.
