Cecil's name is actually Jozette Cecil (ジョゼット・セシル).

Jozette's mother is called Jacqueline (ジャクリーヌ), and Jacqueline is the elder sister of Eugenie (ユージェニー), who is the mother of Guy and his sister, Maribelle (マリィベル).
IOW, Guy and Cecil are cousins in case you haven't figure it out lol.
Another thing to note is that, Maribelle is older than Jozette.

The Cecil family has no male heir, so they adopted (or rather, take in) someone named Gilmore Tinker (ギルモア・ティンカ) as their "son in law" for their eldest daughter.
Note: Gilmore Tinker is a subordinate of Crimson Fabre (Luke's dad).

During the Hod War, the whole Cecil family is labelled as traitors and their rank is taken away from them. They're exiled (or rather, abandoned) by the society, yet they survived with secret support from the Fabre. At 18, Jozette goes up to Crimson Fabre and tells him that she wants to restore her family's name and honor by being a soldier of the Kimulesca military. He accepts and Jozette works directly under him.
Note: She harbours hatred (secretly) towards Ingobert the 6th for sending her aunt, Eugenie, off to Hod and later had it destroyed by the war waged there on purpose.

Jozette has met Guy when he was in the Fabre household, however she didn't know he is her cousin. (She doesn't know it back in Kaitzul either 8D.)

Jozette is about to fall into the Qlipporth and she is saved by Frings.
Jozette: General Frings!
Frings: General Cecil! I will save you right now! Please stay there and don't move!
Jozette: ...wha, what are you talking about!
*Frings saves Hamilton (a subordinate of Jozette) and now it's Jozette's turn.*
Frings: You too, hurry!
Jozette: Do you intend to capture me? If that's what you want, then there's nothing than to be killed by you!
*Jozette draws her sword, but she got subdued.*
Jozette: Le, leave me at once! Now!
*Blah blah blah earth shakes again, Jozette's in danger, blah blah blah.*
Frings: General Cecil!?
*Frings grabs Jozette's arms, but due to Jozette's weight, he got pulled back as well.*
Jozette: ...Your hand...Let go of mine this instance!
Jozette: I have no intention of being insulted by you while I am alive...Let go of me!
Frings: What are you talking about, General Cecil? Do you intend to let Natalia Her Highness worry about you?
Jozette: ......
*Frings pulled her hard enough that the moment she got onto the ground, she ends up inside his embrace.*
Frings: ...It's great that we made it.
*Jozette is so close to Frings that she can hear his heart beat.*
Jozette: You're hurt, if we don't do something about it...
Frings: It's nothing...
Jozette: I have no intention of thanking you.

Anise's Fring's moment.
*After they detained Jozette for the time being. Jade, Natalia, Anise and Frings are talking about whether if they should really let Jozette go.
Because they want to stop both sides from fighting and if they release Jozette, they don't know if she will fight the Markt army for real.*
Anise: I think it's a good idea to release her. I mean, in order to keep your enemy General here, you gotta spend some money on her, right?
Frings: Oh, what a bright young lady! Well then, I will leave it up to you guys.
*Just when Frings turn around and leave...*
Anise: OMG, HE SEZ I AM SMART XDDDD!!11
*Anise puts her hands to her cheeks and turns and twirls while facing Jade.*
Anise: Colonel Jade, is General Frings loaded?
Jade: Why, one would be likely to think so. But I think you are a step too late.
Anise: Hmm? Late? About what?
Tear: Late?
Jade: Oh, nothing at all.
Anise: Ah!
*And Anise seems to get something.*

After Jozette saves Frings from monsters... *
Frings: General Cecil...
Jozette: Do you have something to say?
Frings: N, nothing at all. I, I mean yes.
Jozette: What do you want to say, General Frings?
Frings: ...Please call me Athrun [Arslan].
Jozette: ......
Frings: Oh no, you are mistaken. Um, right about our talk. I would like to propose to you that we cease fire for now.
Jozette: Huh?
Frings: Though this is earlier than what we've had planned, you should be set free very soon.
Jozette: I see...
Frings: You don't look too happy about this.
Jozette: Of course...because I am no longer a captive......
Frings: I am not too happy, but on the subject matter of seeing you being set free, I am......
Frings: Since time running every single moment, I will be frank.
Frings: This year, on the day of my birthday, I received a score from a monestary in Grand Cokuma. The Scoreler said it, I will be having a bride this year.
Jozette: Congratulations.
Jozette: If you have nothing to say, I'll be returning to my room.
Frings: Please wait!
Jozette: You still got sometime left in "this year". I will pray for your good fortune.
Frings: Jozette!
*Frings tries to grab her hand, but she shakes it off.*
Jozette: I can't... No matter how much I like. I can't be with anyone......
Frings: I won't give up! I won't give up!
Jozette: ...Excuse me.
*And Jozette leaves the scene.*

*Some other day, some other time.*
Frings: That bride is you, Jozette.
Jozette: But...
Jozette: But Duke Fabre and I...
*Frings put his finger onto Jozette's lips to signal her to stop.* Frings: Please, don't speak anymore. Your heart is not there, but my heart is......
*Frings takes out a ring.*
Jozette: ...A ring?
Frings: My mother, she died a while ago before I entered the military.
Frings: Before she took her last breath, she said, "Athrun, no matter where you go, where you are, I will be there to protect you".
Frings: Then she took off her ring and clasped it to my hand. There afterwards, I never leave without it.
Jozette: ......
Frings: It's useful. Because of this, I am able to meet you without a scratch.
Frings: I want you to have it.
Jozette: I can't accept this, it's too important for me to accept it!
Frings: Why?
Jozette: Because... Your mother will be sad, wouldn't she?
Frings: No. There is someone out there much more important than myself. I am sure my mother would understand that. Moreover, I want it to protect you.
Jozette: ...No!
Frings: Gah!
Frings: I want to leap over this thing and get to you!
Frings: Jozette, one day this war will end. My feelings are not going to change and I will always be there to wait for you.
Frings: Excuse me!
*Frings leaves.*
Anise: Um, I didn't stand there and listen to everything you guys said 8D;;;
Jozette: ......
Note: The ring is given to Cecil.

Guy's costume, Cool Style is actually the wedding suit for Athrun, but he never had the chance to wear it.

UNCONFIRMED: I am not sure if this is true, but something weird that I picked up (maybe misread?) is that, it seems Duke Fabre has something for Jacqueline and Jozette......