In the picaresque novels the heroine was beautiful and was the idol of the hero of the story. Cunégonde has the same image to Candide and she is similarly adored by him.  However, Voltaire, satirising the picaresque tradition, gives his heroine very human failings.

Cunégonde is beautiful.
We have a description of her in Chapter 1 page 56 
Sa fille Cunégonde, âgée de dix-sept ans, était haute en couleur, fraîche, grasse, appétissante. 
She was a little on the plump side but none the worse for that.  She quickly attracted the attention and won the devotion of the men she came across.  When she gave herself to the Governor of Buenos Aires, she soon supplanted his other women to become his favourite mistress.

Cunégonde is sensual
Her name is sexually suggestive.  After she had been excited by seeing Pangloss making love to Paquette, she took the lead in seducing Candide by dropping her handkerchief near to him. She remained physically attracted to him.  When she watched Candide being flogged at the Auto da fé the white skin of his naked body gave her a thrill.
Forgetting herself, she mentions that the Bulgare officer, who made her his mistress, was handsome and he also had nice white skin.  
She seemed quite comfortable to be the mistress of two men at the same time in Lisbon and her only concern mentioned was the problem of timetabling. She claimed to have resisted the advances but we note that they gave her a lot of money and jewellery for enjoying her celibacy.

Cunégonde is pleasure loving and appreciates status and luxury
She enjoys the opulent lifestyle afforded to her by the generosity of the Jewish merchant, Don Issachar, and the Grand Inquisitor and laments the loss after all the wealth they had provided for her has been stolen. Chapter 10 page 76:
«Qui a donc pu me voler mes pistoles et mes diamants?' disait en pleurant Cunégonde; de quoi vivrons-nous? Comment ferons- nous? Où trouver des inquisiteurs et des juifs qui m'en donnent d'autres?

She is honoured to be given a good seat to view the executions at the Auto da fé and appreciates the quality of the refreshments served between the mass and the execution. - She enjoys good food.

Cunégonde is not steadfast in her love.
On arrival in Buenos Aires she is quickly tempted by the insidious charms of the Governor.  When told that the officers of the law are coming to arrest Candide, there is no real question of her making a quick escape with him to face unknown hardships together.  When Don Fernando  d'Ibaraa, y Figueora, y Mascarenes, y Lampourdos, y Souza propositions her, Cunégonde asks for a quarter of an hour to make up her mind and the old lady illustrates the high moral plane on which her decision will be made.  Chapter 13 page 86:
…il ne tient qu'à vous d'être la femme du plus grand seigneur de l'Amérique méridionale, qui a une très belle moustache; 
Predictably, Cunégonde stays in Buenos Aires to be the Governor's mistress.

The role that Cunégonde plays in the book.
The theme of “Candide” is the validity of the Optimistic view of life.  Cunégonde is the light of optimism that inspires Candide throughout.  In his direst misfortunes, Candide is fortified by the belief that one day he will be reunited with the beautiful woman he loves.
This is a great irony because the events of the book tell us that Cunégonde is amoral, selfish, materialistic and disloyal and Candide’s idealised love is an illusion.  Her role in the book is to show that romance is just another optimistic illusion that will lead eventually to disappointment.

To reinforce this point, the author shows how the process of ageing took away her beauty and destroyed her charm   Candide no longer felt any love for her.  He decided to marry her, although he had no wish to, because she was pressing him and he wished to affront the insolence of the Baron. With time she became even uglier and more shrewish.

Some critics have pointed out that this harsh transformation was perhaps improbable, because not long before this, her charms were sufficient to attract the proud and rich governor of Buenos Aires.  However to have an ending where Candide was reunited with his beloved mistress would have made this a totally different book and would have given the message that everything turns out fine in the end.  This was not the view of life that Voltaire wished to convey.

A more positive conclusion to this character study is to record how Cunégonde adapted finally to fit into Candide’s hardworking team.  Chapter 30 page 150
Cunégonde était, à la vérité, bien laide, mais elle devint une excellente pâtissière;