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Vangandr Hmm. I write bios for all my dragons, so I guess I will describe how I do it? Maybe that will help give you ideas. There are plenty of examples if you visit my lair - all my dragons have bios, although I'm redoing the formatting so the ones on the second page aren't changed over yet.
I start with three descriptive words - such as "Kind, Intelligent, Gentle." This provides a framework for the dragon's personality. I like randomizing these traits on a site like
this for ideas for new dragons (although I usually only actually use one or two I like from the random result and pick something that complements it well as the third). Dragons with parents in the clan inherit two random traits from the traits of their parents and then gain an additional random one.
Since every dragon that enters into my lair does so as a hatchling, I then give a brief description of where they come from. For example: Was the egg found abandoned in the woods? Was the dragon the first or secondborn child of such and such other dragons? Just a little sentence or two about where they came from.
Then I have my bios divided into three sections after that: Hatchling Bio, Adolescent Bio, and Adult Bio.
For the Hatchling Bio, I describe what the dragon is like as a baby. This is easy because baby dragons like playing and chewing on sticks and such, but each dragon is a bit different. Some like running around, some like swimming, some like drawing or crafts. Some are scared of everything and some are reckless. I usually just write a little paragraph or a couple of sentences for each of these bio sections.
Then, when that dragon reaches 6 days and grows to adolescence, I write the Adolescent Bio. In this bio, I start to turn the dragon's baby personality into an older personality, describing how they have changed as they have aged. For example, a baby that has a lot of energy and chases things might become a great hunter. A little dragon that likes building things with sticks and rocks might become an engineer, craftsdragon, or architect, etc.
Once that dragon reaches the end of their first cooldown (15 days for plentifuls, etc.) I write the Adult Bio. In this section, I describe what they are like as an adult: what is their job, their charge (if a guardian), their mate, etc. Since there is so much time between each of these three life stages, I have plenty of time to think between them and don't have to fret about writing them all at once.
Many dragons have extra paragraphs after that describing key life events: such as starting a family or a life-changing injury - but those are added on much later as stuff happens. I'm a Nuzlocke-ish lair, so there's a lot of stuff going on.