Immortality comes up a lot in fiction.  These timeless beings turn up as villains, heroes, love interests, cursed secondary characters, and everything in-between.  Many people hate a protagonist with this power because it’s seen as a sign of perfection.  They live forever at a perfect age with no physical flaws and an intelligence that allows them to absorb all that time has to offer.  To be fair, one could think a person who has been given immortality would also have the natural ability to evolve with time.  Then again, they are still human in some way, so giving them Eidetic memory too might be going too far.  So, what are some ways to make your immortals work?

1. Even though they live forever, you should have some way for them to die or be removed from the story.  There needs to be a threat to these characters.  Maybe a special item can undo their powers or another immortal can do the job.  Trapped for eternity can work as well.  This is what happened to the Baron in Legends of Windemere and even Dariana to some extent.  The real limit on her is that she can still be knocked out, injured, possibly killed, or overwork her body by using her powers.
2. A physical flaw can help in the removal of the ‘perfection’ tag.  A scar or missing body part can be explained as happening prior to immortality or the price.  It doesn’t even have to visible, but be part of a big reveal later.  Maybe the character is ashamed and has found a way to hide it someway.  I will admit that I didn’t really do this with Dariana since she uses her telepathy to limit the attention she gets.
3. If the immortal is your protagonist then you need to factor in their long life.  They should have a lot of knowledge and maybe get confused at times.  After a couple hundred years, you get your social etiquette mixed up.  Is it still right to high five or is that old news?  What phrases no longer mean what they used to mean?  Even after being in multiple books, Dariana still isn’t sure about her reactions.  She uses old greetings and blessings instead of the more casual interactions of today.
4. This might be an odd one, but you need to keep in mind that immortals could lose any concept of time.  They could always be late or never know what day it is.  Maybe they even lack the knowledge of the year because they work off a different calendar.  This has mostly comedy effects.
5. Building off the previous idea, an immortal could have an apathy toward life events like marriages, births, funerals, and other milestones.  A mortal will only have about 100 years to have all of these happen, which gives the more impact.  Somebody who has gone through hundreds of these events will either not care or try to fake it for those around them.  Honestly, there are people my age that have become emotionally numb to life, so immortals would have a higher risk of this.
6. Like any character, these types should have some kind of goal.  There has to be something they want even after centuries of life.  For example, Dariana wants friends and to see the world instead of being put back to sleep.  There are a few other things that are spoilers, so read The Spirit Well to find out.
7. Finally: Insanity is always an option.  This explains Yola Biggs and Stephen Kernaghan perfectly.  Dariana . . . read The Spirit Well to find out!