how to build a character and make them better?

I can't answer the second, but I can answer the first

let the creation work in tandem
trust your gut to tell you who this is
who this was
who this can be

learn who they are instead of telling them
work by chance
do what feels right, not what you had planned
even the silly moments are sweet
and sometimes mean more than you know

remember that this is a person
or a place or a creature
not just a projection of yourself
nothing wrong with that, of course
but to create someone new you need to meet them
not just build them from a blueprint

listen to your heart
and to the songs they play in your mind
draw out your deeper parts
and put them on display

let bad things happen to them
but good ones, too
even if one outweighs the other
never break the scale
never light every candle
and never shutter every lamp

let them find a purpose that feels right
and let them fight for it
help them do what needs to be done
and help them grieve when things are lost

but most importantly, remember
every story has an end
and though it may last for centuries on end
someday remember to say goodbye