Generally when they are referred to in Buddhist literature it is assumed they are devoted to the Truth (Dharma). But the broader meaning of the word is a female (males are called Dakas) spirit who has some freedom but is still bound by their past actions. Thus there are good Angels and bad (or simply confused) Angels. Some people might argue that you couldn't call them an Angel anymore if they're bad, but that's ridiculous since being predictable is not part of anyones long-term nature.
Thus there are worldy Dakinis who are malicious and use their powers to confuse us, to arouse passions that are destructive, and who use their charms for their own selfish purposes. Dakinis can manifest on earth in human form or take birth as a human. These are then called "worldly Dakinis" and I'm sure you've met at least one or two in your lifetime. Women (or men) who seem to be able to simply will things into being. People who are so charming that they seduce whatever they want from the world and enjoy it fully, yet also remain somewhat disattached from the results.
Dharma Dakinis are devoted to a higher purpose and so their actions are generally positive. But it is hard to generalize the actions of Dharma Dakinis since they aim to break our rabid fixation on concrete thoughts. Dharma Dakinis are not afraid to use their powers to dazzle us, arouse us out of our sleep of habits, and seduce us onto a path of truth. Dakinis are tricky, capricious, and unpredictable. We're in trouble when the Dakini disrupts our mind with magic, breaks our habitual thought patterns with the miraculous, or simply opens our hearts with mad adoration. Messengers between our earthly realm and the higher realms of the teachers, they help to bring the powerful teachings to earth, protect the truth from destructive forces, and bring blessings to sincere seekers. The natural result is to fall in love with a Dakini or Angel. How can you avoid falling in love with someone who inspires wild love and higher meaning suffused with delight?
Fixate on that pleasure and the Dakini does the most compassionate thing possible. They fly away. Dakinis do not manifest simply for our pleasure. (There are people who will do that but it usually requires negotiation and some form of hard currency). Dakinis manifest the beauty of the Truth which is then an invitation to follow the path of Truth. Sky Dancers help us through vision and inspiration. But although we can be greatly inspired by a Dakini (another word is Muse), they will only come back if we put into action what they have shown us.
In that regard a Dakini's love is unswerving and always compassionate. It doesn't matter if we wait a few hours or a few lifetimes to put our inspiration into action. Miracles occur but sometimes it takes years to fully realize the implications. When we do realize or remember, our hearts open, and again the magic and majesty of the universe is open to us. The Dakinis celebrate the Truth and our resolve to work for it in a most extraordinary way. They dance in the sky.