Phasma, as she herself pointed out in her comic issue, is a survivor. She places her life and her goals above all of the others, making her a person who will sacrifice even people who trusts her, for saving herself or gaining something. People usually have seen her as terrifying officer, as fanatically devoted to First Order cause, as her troops, who she heavily indoctrinated since young years. But little First Order meant for her, if she had seen better opportunity or her life was in danger. That showed best in her comics, when she sacrificed the colonists of the alien planet, to pass way through the monsters swarm. Or, in the The Force Awakens, when she without hesitation shut down Starkiller’s shields, threatened by Finn or killed the stormtroopers who knew about her betrayal.
That, though, in my eyes, doesn’t make her a coward. She charges to battle and uses her commanding and fighting skills as eagerly. She is more of a self-serving person, who climbs on top over the dead bodies, rather than a coward. She thinks her life is too important to sacrifice it. She was scheming and plotting to get there, so she won’t “leave loose ends behind”.
She wouldn’t find acceptance, if she was open with it, so she puts her helmet and pretends. Pretends her troops life is important for her, pretends she follows First Order and sacrifices others’ life only when there is no others choice.
She easily gains hearts of her troops, they see her as battle goddes of some sorts. She has way with speeches and her motivational talk is always very inspiring. That, given her aura of mystery, aided by her chrome armor and terrifying efficiency in what she does, makes her a a person to look up to.
She cares about all of this, this feeds her self-serving nature. But she probably, knowing that First Order is not anymore the most powerful force on the galaxy, would just leave them. To again serve only herself, leading herself into well chosen battles.
This all make her a very dangerous person to deal with. She will not care about you, when you fell in the battle and will kill you, only if you saw her failing. Removing weak links is her speciality and all who knows about her nature are such links.
Phasma still managed to train and send into battle whole armies of troops who admire her. Instill fear in hearts of enemies. This just proves how lie told many times becomes truth.