Grace.
Inara is graceful. That's obvious. Even when awakened from sleep by Jubal Early, she still looks more composed than any other crew member at their best.
Her grace is not only physical, but mental. To be able to meet the needs of numerous clients while not becoming emotionally attached, she compartmentalizes her life. Her time, her emotions, her appearance are kept on schedules to avoid mental slip ups.
Her gracefulness make people desire her, but it is her graciousness that makes them confide in her. When people are genuine with Inara, she's genuine right back. When they are false, she keeps them at arm's length. On the one hand, look at Saffron and Jayne... they wonder how they can manipulate Inara and so she's always on guard against them. As for Mal, he never tries to manipulate her, but he's not honest with her... and so she's not honest with him. Even when he's facing certain death (Shindig), he won't open up to her. When she met Book, she was on guard, but his humility and honesty quickly gained her respect... despite the fact they had differing religions. She gave Book grace when he desperately needed it.
Kaylee is a breath of fresh air for Inara. Inara's friends have always been other companions trained in psychology, situations, emotions, culture and sex. But Kaylee knows none of these things. She genuinely wants to be Inara's friend. So instead of perfecting her nails, makeup, and hair for status or clients, Inara finds herself doing it for fun with Kaylee. Kaylee's love and admiration for Inara are genuine, so Inara is fiercely protective of her.
Mal always kept her at arms length. But on one occasion he seemed be "truthsome" with her and she responded in kind. After the crew escaped their first encounter with Saffron, Mal talked to Inara about her gracefulness. She was immediately on guard, but when Mal pressed her to admit that she did not fall she was ready to be honest with him. But Mal's biggest problem was that he's a man, i.e. an idiot.
Inara also represents grace under pressure. The two enigmas of the crew were Book's past and Inara's future. If the series had gone longer, we'd have learned that Inara is terminally ill.
When they think the Reavers are going to board them, Inara opens a box with a syringe. That's either an enormous dose of poison or treatment for her condition.
When they think they're going to suffocate, Simon laments that he's going to die on his birthday... to which Inara mutters, "I don't want to die at all."
In Heart of Gold, Nandi can't figure out why Inara left the central planets. Inara was destined to be a high priestess or Pope of the Pillow or somesuch.
And at the end of Heart of Gold, when Mal finally opens up to Inara, she thanks him for his courage and determination. She tells him she's getting addicted to it and that's why she's going to leave.
Why would she leave when THE man finally starts be real with her? Why would she shut him down after we've seen her kiss him and cry over his love life? The reason is that she does not have long to live and she does not want to abandon him after he finally admits his love. She kept her feelings for him in a separate compartment. When he finally started to open it, she slammed it shut.
So how long could Inara have kept her job, her illness, her friendships, her enmities, her dashed hopes, and her unspoken love in separate emotional boxes? Not forever. The graceful woman would have had a meltdown of epic proportions sooner or later. And then we'd have seen how the gracious woman accepts grace in return.
