I thought about how the main quest for Yvette would look like. Of course you would do 4 sidequests to get her to trust the player (which start/play in different locations like Café Théâtre and Sorbonne) and here is my little summary.
The main quest with her plays in year 1795, around a year after Arno has been in Franciade.
-Sylaiises

A savior in need

Danger calls. Instead of fulfilling the Templars wish, Arno needs to follow her lead first.
Free the former royalist informant of Mademoiselle Montmorency to win her trust!

Take back what is yours

Mademoiselle Montmorency asks for Arnos help in the Café. 
A few remaining jacobins of Ropespierres broke into her office in Quartier Latin.
Get the documents that got stolen by the jacobins.

A strong, right hand

The spymaster inflicted the rage of former people of hers. They allianced with a few royalists.
Support Mademoiselle Montmorency in the fight against the them.

A bodyguard for any case

It is not safe for one of her supporters and Mademoiselle Montmorency asks Arno to look out
for the people, that need to leave Paris as soon as possible. 
Le Havre is not save enough, so they need to get the next carriage to Franciade.
Collect the family members of an informants family and protect them on their way out of Paris.

(BBC The Musketeers - Ninon de Larroque)

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The player starts at Sorbonne in Quartier latin waiting for Yvette to arrive. Waiting is useless, as the young Mademoiselle doesn't appear, which is unusual - the player should know that Yvette is an overpunctual person (she always waits for the player when doing the sidequests) and most likely appears before the player arrives. The protagonist Arno looks worried on his clock. Steps appear in front of him and he is ready to meet the Mademoiselle, but instead the assassin meets her older brother Mathis. The aged man reports that their father got behind the plan of murder from his sister and threatens to kill his own child. 

Yvettes betrayal did not went as planned and her father threatens now her life. In order to obtain powerful connections within the Templar order, you need to save her from the imprisonment. Immediately the player leaves Mathis presence to save the Templar informant.

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QUEST

Protect the Templar spymaster

Ø Free the area from the opponents
Ø Find the key for the prison
Ø Free the prisoner
Ø Eliminate Thomaj Montmorency

Optional

Ø Assassination from behind a cover (0/4)
Ø Sabotage alarm bells. (0/2)
Ø Obtain the family medaillon to keep the secret of the Croy family save.

Location of Action

Thomaj Montmorency keeps Yvette in the Abbey of Saint-Germain-de-Prés. Your mission is to free the restricted area from all jacobins and obtain the key for the prison. While doing that you need to fulfill the optional conditions to get the quest to 100%
As soon as you, the player, killed all 24 jacobins and obtained the key, you need to find a way into the catacombs below Saint-German-de-Prés. You may need to picklock a few doors. 

Following dialogue can be heard, if you do not move for a while and stay in the shadows:

(Yvette> Y. // Thomaj> T. )

Y: Father, what is this madness! You will not succeed with this plan.
T: An ill-bred child like you should not even start this discussion, Yvette. You are not in the position to even speak with me!
Y: Whatever you plan to do, be sure, that someone will be worried, even if its the smallest, the poorest person in bien-áime Paris.
T: Be silent!
Y: They all care about me, as I care about them! And they are courageous. They will rescue me, united as one. I am like Theroigne, leading the people that nobody wants or cares about. I teached them a lot, wisdom and knowledge from the books I read.

-silence follows, before Thomaj walks towards the cell-

T: I never understood why you read so much. Diving into fanatic stories of mad men, enjoying literature, like a grown man or politician should. It was a mistake, that I even allowed your mother to let you read. All those phantasms drove you into this state of yours. This is all your fault, it is not mine!
Y: Those book opened my eyes, where others decided to close them! If theres a lunatic present in this room, it is you, who allowed the mentor to beat his own child, molest and forced it to actions she never wanted to do. 

-Yvettes rattles the cells door, the player hears her snivel-

Y: You once said, that every action makes us to what we are. I say that every action forms you like a sculptor forms the marble, I say that every single mistake carves into your flesh and soul like only a dagger can. And if I look into your face, I see uncountable scars and fleshwounds - its makes you uglier than Louis Carpet's detached head.
T: I should silence you now and not wait for the execution master!

The player jumps into the conversation, both Templars are surprised. Arno reacts to slow to kill Thomaj at first sight and so the father of your informant flees. You open the cell door and find a distressed young woman, close to tears.

Following dialogue can be heard, if you do not skip the cutscene:

(Arno>A. // Yvette> Y))

Y: Monsieur, you can't imagine, how glad I am to see you.
A: Oh, I can.

- The assassin smirks-

Y: Do not let that rescue go to your head. Give me a belt and a sword and I shall fight by your side.
A: Mademoiselle, are you sure you can fight? The imprisonment...?
Y: My skills may not reach yours, but I am assured that I can kill my father. And I've been in there for more or less a day. I took no harm.
A: You gave me, the assassin, the order to eliminate your father.
Y: And you failed, obviously. Instead of killing him with your infamous phantom blade, you freed me from my shackles.
A: Your safety was important.

-Silence between the two follows, Arno remembers Élise and her decision of taking revenge, Yvette seems to be much like her-

A: Not only for me, but for the brotherhood as well. Your net of informants, already forgot that?
Y: My brother has the same rights as I with my people. I am replaceable, assassin, and I am only one of many.
A: Everyone has the same value, no matter what alliance or how many riches he has. You are not replaceable.

-Yvettes places her index fingers upon his chest, silence him with a serious look on her face-

Y: All a sight of perspective.

-He takes her hand, places a soft kiss on the back of it-

A: Maybe, but I believe that my perspective is the legit one.

-She sighs, taking her hand back while rolling with her eyes-

Y: Now, theres one thing I must ask of you.
A: I live to serve, Mademoiselle. How may I help you?

-She chuckles about the saying of his-

Y: You remember my choker. My father stole it from me. It is a heirloom of my mother and her family and I want it back. There are secrets in this medaillon, that should never see the daylight.
A: I will look out for it, I promise. Shall we leave this cursed place?
Y: Go ahead. I will follow.